1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
|
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<meta name="nodejs.org:node-version" content="v22.14.0">
<title>Async hooks | Node.js v22.14.0 Documentation</title>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback">
<link rel="stylesheet" href="assets/style.css">
<link rel="stylesheet" href="assets/hljs.css">
<link rel="canonical" href="https://nodejs.org/api/async_hooks.html">
<script async defer src="assets/api.js" type="text/javascript"></script>
<script>
const storedTheme = localStorage.getItem('theme');
// Follow operating system theme preference
if (storedTheme === null && window.matchMedia) {
const mq = window.matchMedia('(prefers-color-scheme: dark)');
if (mq.matches) {
document.documentElement.classList.add('dark-mode');
}
} else if (storedTheme === 'dark') {
document.documentElement.classList.add('dark-mode');
}
</script>
<style>@media(max-width:606px){.with-48-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:542px){.with-40-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:1072px){.with-72-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:1048px){.with-69-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:1144px){.with-81-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:590px){.with-46-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:638px){.with-52-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:1080px){.with-73-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}@media(max-width:1176px){.with-85-chars>.js-flavor-toggle{float:none;margin:0 0 1em auto;}}</style>
</head>
<body class="alt apidoc" id="api-section-async_hooks">
<a href="#apicontent" class="skip-to-content">Skip to content</a>
<div id="content" class="clearfix">
<div role="navigation" id="column2" class="interior">
<div id="intro" class="interior">
<a href="/" title="Go back to the home page">
Node.js
</a>
</div>
<ul>
<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li>
<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li>
</ul>
<hr class="line">
<ul>
<li><a href="assert.html" class="nav-assert">Assertion testing</a></li>
<li><a href="async_context.html" class="nav-async_context">Asynchronous context tracking</a></li>
<li><a href="async_hooks.html" class="nav-async_hooks active">Async hooks</a></li>
<li><a href="buffer.html" class="nav-buffer">Buffer</a></li>
<li><a href="addons.html" class="nav-addons">C++ addons</a></li>
<li><a href="n-api.html" class="nav-n-api">C/C++ addons with Node-API</a></li>
<li><a href="embedding.html" class="nav-embedding">C++ embedder API</a></li>
<li><a href="child_process.html" class="nav-child_process">Child processes</a></li>
<li><a href="cluster.html" class="nav-cluster">Cluster</a></li>
<li><a href="cli.html" class="nav-cli">Command-line options</a></li>
<li><a href="console.html" class="nav-console">Console</a></li>
<li><a href="corepack.html" class="nav-corepack">Corepack</a></li>
<li><a href="crypto.html" class="nav-crypto">Crypto</a></li>
<li><a href="debugger.html" class="nav-debugger">Debugger</a></li>
<li><a href="deprecations.html" class="nav-deprecations">Deprecated APIs</a></li>
<li><a href="diagnostics_channel.html" class="nav-diagnostics_channel">Diagnostics Channel</a></li>
<li><a href="dns.html" class="nav-dns">DNS</a></li>
<li><a href="domain.html" class="nav-domain">Domain</a></li>
<li><a href="errors.html" class="nav-errors">Errors</a></li>
<li><a href="events.html" class="nav-events">Events</a></li>
<li><a href="fs.html" class="nav-fs">File system</a></li>
<li><a href="globals.html" class="nav-globals">Globals</a></li>
<li><a href="http.html" class="nav-http">HTTP</a></li>
<li><a href="http2.html" class="nav-http2">HTTP/2</a></li>
<li><a href="https.html" class="nav-https">HTTPS</a></li>
<li><a href="inspector.html" class="nav-inspector">Inspector</a></li>
<li><a href="intl.html" class="nav-intl">Internationalization</a></li>
<li><a href="modules.html" class="nav-modules">Modules: CommonJS modules</a></li>
<li><a href="esm.html" class="nav-esm">Modules: ECMAScript modules</a></li>
<li><a href="module.html" class="nav-module">Modules: <code>node:module</code> API</a></li>
<li><a href="packages.html" class="nav-packages">Modules: Packages</a></li>
<li><a href="typescript.html" class="nav-typescript">Modules: TypeScript</a></li>
<li><a href="net.html" class="nav-net">Net</a></li>
<li><a href="os.html" class="nav-os">OS</a></li>
<li><a href="path.html" class="nav-path">Path</a></li>
<li><a href="perf_hooks.html" class="nav-perf_hooks">Performance hooks</a></li>
<li><a href="permissions.html" class="nav-permissions">Permissions</a></li>
<li><a href="process.html" class="nav-process">Process</a></li>
<li><a href="punycode.html" class="nav-punycode">Punycode</a></li>
<li><a href="querystring.html" class="nav-querystring">Query strings</a></li>
<li><a href="readline.html" class="nav-readline">Readline</a></li>
<li><a href="repl.html" class="nav-repl">REPL</a></li>
<li><a href="report.html" class="nav-report">Report</a></li>
<li><a href="single-executable-applications.html" class="nav-single-executable-applications">Single executable applications</a></li>
<li><a href="sqlite.html" class="nav-sqlite">SQLite</a></li>
<li><a href="stream.html" class="nav-stream">Stream</a></li>
<li><a href="string_decoder.html" class="nav-string_decoder">String decoder</a></li>
<li><a href="test.html" class="nav-test">Test runner</a></li>
<li><a href="timers.html" class="nav-timers">Timers</a></li>
<li><a href="tls.html" class="nav-tls">TLS/SSL</a></li>
<li><a href="tracing.html" class="nav-tracing">Trace events</a></li>
<li><a href="tty.html" class="nav-tty">TTY</a></li>
<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li>
<li><a href="url.html" class="nav-url">URL</a></li>
<li><a href="util.html" class="nav-util">Utilities</a></li>
<li><a href="v8.html" class="nav-v8">V8</a></li>
<li><a href="vm.html" class="nav-vm">VM</a></li>
<li><a href="wasi.html" class="nav-wasi">WASI</a></li>
<li><a href="webcrypto.html" class="nav-webcrypto">Web Crypto API</a></li>
<li><a href="webstreams.html" class="nav-webstreams">Web Streams API</a></li>
<li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li>
<li><a href="zlib.html" class="nav-zlib">Zlib</a></li>
</ul>
<hr class="line">
<ul>
<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li>
</ul>
</div>
<div id="column1" data-id="async_hooks" class="interior">
<header class="header">
<div class="header-container">
<h1>Node.js v22.14.0 documentation</h1>
<button class="theme-toggle-btn" id="theme-toggle-btn" title="Toggle dark mode/light mode" aria-label="Toggle dark mode/light mode" hidden>
<svg xmlns="http://www.w3.org/2000/svg" class="icon dark-icon" height="24" width="24">
<path fill="none" d="M0 0h24v24H0z" />
<path d="M11.1 12.08c-2.33-4.51-.5-8.48.53-10.07C6.27 2.2 1.98 6.59 1.98 12c0 .14.02.28.02.42.62-.27 1.29-.42 2-.42 1.66 0 3.18.83 4.1 2.15A4.01 4.01 0 0111 18c0 1.52-.87 2.83-2.12 3.51.98.32 2.03.5 3.11.5 3.5 0 6.58-1.8 8.37-4.52-2.36.23-6.98-.97-9.26-5.41z"/>
<path d="M7 16h-.18C6.4 14.84 5.3 14 4 14c-1.66 0-3 1.34-3 3s1.34 3 3 3h3c1.1 0 2-.9 2-2s-.9-2-2-2z"/>
</svg>
<svg xmlns="http://www.w3.org/2000/svg" class="icon light-icon" height="24" width="24">
<path d="M0 0h24v24H0z" fill="none" />
<path d="M6.76 4.84l-1.8-1.79-1.41 1.41 1.79 1.79 1.42-1.41zM4 10.5H1v2h3v-2zm9-9.95h-2V3.5h2V.55zm7.45 3.91l-1.41-1.41-1.79 1.79 1.41 1.41 1.79-1.79zm-3.21 13.7l1.79 1.8 1.41-1.41-1.8-1.79-1.4 1.4zM20 10.5v2h3v-2h-3zm-8-5c-3.31 0-6 2.69-6 6s2.69 6 6 6 6-2.69 6-6-2.69-6-6-6zm-1 16.95h2V19.5h-2v2.95zm-7.45-3.91l1.41 1.41 1.79-1.8-1.41-1.41-1.79 1.8z"/>
</svg>
</button>
</div>
<div id="gtoc">
<ul>
<li class="pinned-header">Node.js v22.14.0</li>
<li class="picker-header">
<a href="#toc-picker" aria-controls="toc-picker">
<span class="picker-arrow"></span>
Table of contents
</a>
<div class="picker" tabindex="-1"><div class="toc"><ul id="toc-picker">
<li><span class="stability_1"><a href="#async-hooks">Async hooks</a></span>
<ul>
<li><a href="#terminology">Terminology</a></li>
<li><a href="#overview">Overview</a></li>
<li><a href="#async_hookscreatehookcallbacks"><code>async_hooks.createHook(callbacks)</code></a>
<ul>
<li><a href="#error-handling">Error handling</a></li>
<li><a href="#printing-in-asynchook-callbacks">Printing in <code>AsyncHook</code> callbacks</a></li>
</ul>
</li>
<li><a href="#class-asynchook">Class: <code>AsyncHook</code></a>
<ul>
<li><a href="#asynchookenable"><code>asyncHook.enable()</code></a></li>
<li><a href="#asynchookdisable"><code>asyncHook.disable()</code></a></li>
<li><a href="#hook-callbacks">Hook callbacks</a>
<ul>
<li><a href="#initasyncid-type-triggerasyncid-resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a>
<ul>
<li><a href="#type"><code>type</code></a></li>
<li><a href="#triggerasyncid"><code>triggerAsyncId</code></a></li>
<li><a href="#resource"><code>resource</code></a></li>
<li><a href="#asynchronous-context-example">Asynchronous context example</a></li>
</ul>
</li>
<li><a href="#beforeasyncid"><code>before(asyncId)</code></a></li>
<li><a href="#afterasyncid"><code>after(asyncId)</code></a></li>
<li><a href="#destroyasyncid"><code>destroy(asyncId)</code></a></li>
<li><a href="#promiseresolveasyncid"><code>promiseResolve(asyncId)</code></a></li>
</ul>
</li>
<li><a href="#async_hooksexecutionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li>
<li><a href="#async_hooksexecutionasyncid"><code>async_hooks.executionAsyncId()</code></a></li>
<li><a href="#async_hookstriggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li>
<li><a href="#async_hooksasyncwrapproviders"><code>async_hooks.asyncWrapProviders</code></a></li>
</ul>
</li>
<li><a href="#promise-execution-tracking">Promise execution tracking</a></li>
<li><a href="#javascript-embedder-api">JavaScript embedder API</a>
<ul>
<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a></li>
</ul>
</li>
<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a></li>
</ul>
</li>
</ul></div></div>
</li>
<li class="picker-header">
<a href="#gtoc-picker" aria-controls="gtoc-picker">
<span class="picker-arrow"></span>
Index
</a>
<div class="picker" tabindex="-1" id="gtoc-picker"><ul>
<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li>
<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li>
<li>
<a href="index.html">Index</a>
</li>
</ul>
<hr class="line">
<ul>
<li><a href="assert.html" class="nav-assert">Assertion testing</a></li>
<li><a href="async_context.html" class="nav-async_context">Asynchronous context tracking</a></li>
<li><a href="async_hooks.html" class="nav-async_hooks active">Async hooks</a></li>
<li><a href="buffer.html" class="nav-buffer">Buffer</a></li>
<li><a href="addons.html" class="nav-addons">C++ addons</a></li>
<li><a href="n-api.html" class="nav-n-api">C/C++ addons with Node-API</a></li>
<li><a href="embedding.html" class="nav-embedding">C++ embedder API</a></li>
<li><a href="child_process.html" class="nav-child_process">Child processes</a></li>
<li><a href="cluster.html" class="nav-cluster">Cluster</a></li>
<li><a href="cli.html" class="nav-cli">Command-line options</a></li>
<li><a href="console.html" class="nav-console">Console</a></li>
<li><a href="corepack.html" class="nav-corepack">Corepack</a></li>
<li><a href="crypto.html" class="nav-crypto">Crypto</a></li>
<li><a href="debugger.html" class="nav-debugger">Debugger</a></li>
<li><a href="deprecations.html" class="nav-deprecations">Deprecated APIs</a></li>
<li><a href="diagnostics_channel.html" class="nav-diagnostics_channel">Diagnostics Channel</a></li>
<li><a href="dns.html" class="nav-dns">DNS</a></li>
<li><a href="domain.html" class="nav-domain">Domain</a></li>
<li><a href="errors.html" class="nav-errors">Errors</a></li>
<li><a href="events.html" class="nav-events">Events</a></li>
<li><a href="fs.html" class="nav-fs">File system</a></li>
<li><a href="globals.html" class="nav-globals">Globals</a></li>
<li><a href="http.html" class="nav-http">HTTP</a></li>
<li><a href="http2.html" class="nav-http2">HTTP/2</a></li>
<li><a href="https.html" class="nav-https">HTTPS</a></li>
<li><a href="inspector.html" class="nav-inspector">Inspector</a></li>
<li><a href="intl.html" class="nav-intl">Internationalization</a></li>
<li><a href="modules.html" class="nav-modules">Modules: CommonJS modules</a></li>
<li><a href="esm.html" class="nav-esm">Modules: ECMAScript modules</a></li>
<li><a href="module.html" class="nav-module">Modules: <code>node:module</code> API</a></li>
<li><a href="packages.html" class="nav-packages">Modules: Packages</a></li>
<li><a href="typescript.html" class="nav-typescript">Modules: TypeScript</a></li>
<li><a href="net.html" class="nav-net">Net</a></li>
<li><a href="os.html" class="nav-os">OS</a></li>
<li><a href="path.html" class="nav-path">Path</a></li>
<li><a href="perf_hooks.html" class="nav-perf_hooks">Performance hooks</a></li>
<li><a href="permissions.html" class="nav-permissions">Permissions</a></li>
<li><a href="process.html" class="nav-process">Process</a></li>
<li><a href="punycode.html" class="nav-punycode">Punycode</a></li>
<li><a href="querystring.html" class="nav-querystring">Query strings</a></li>
<li><a href="readline.html" class="nav-readline">Readline</a></li>
<li><a href="repl.html" class="nav-repl">REPL</a></li>
<li><a href="report.html" class="nav-report">Report</a></li>
<li><a href="single-executable-applications.html" class="nav-single-executable-applications">Single executable applications</a></li>
<li><a href="sqlite.html" class="nav-sqlite">SQLite</a></li>
<li><a href="stream.html" class="nav-stream">Stream</a></li>
<li><a href="string_decoder.html" class="nav-string_decoder">String decoder</a></li>
<li><a href="test.html" class="nav-test">Test runner</a></li>
<li><a href="timers.html" class="nav-timers">Timers</a></li>
<li><a href="tls.html" class="nav-tls">TLS/SSL</a></li>
<li><a href="tracing.html" class="nav-tracing">Trace events</a></li>
<li><a href="tty.html" class="nav-tty">TTY</a></li>
<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li>
<li><a href="url.html" class="nav-url">URL</a></li>
<li><a href="util.html" class="nav-util">Utilities</a></li>
<li><a href="v8.html" class="nav-v8">V8</a></li>
<li><a href="vm.html" class="nav-vm">VM</a></li>
<li><a href="wasi.html" class="nav-wasi">WASI</a></li>
<li><a href="webcrypto.html" class="nav-webcrypto">Web Crypto API</a></li>
<li><a href="webstreams.html" class="nav-webstreams">Web Streams API</a></li>
<li><a href="worker_threads.html" class="nav-worker_threads">Worker threads</a></li>
<li><a href="zlib.html" class="nav-zlib">Zlib</a></li>
</ul>
<hr class="line">
<ul>
<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li>
</ul></div>
</li>
<li class="picker-header">
<a href="#alt-docs" aria-controls="alt-docs">
<span class="picker-arrow"></span>
Other versions
</a>
<div class="picker" tabindex="-1"><ol id="alt-docs"><li><a href="https://nodejs.org/docs/latest-v23.x/api/async_hooks.html">23.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v22.x/api/async_hooks.html">22.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v21.x/api/async_hooks.html">21.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v20.x/api/async_hooks.html">20.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v19.x/api/async_hooks.html">19.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v18.x/api/async_hooks.html">18.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v17.x/api/async_hooks.html">17.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v16.x/api/async_hooks.html">16.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v15.x/api/async_hooks.html">15.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v14.x/api/async_hooks.html">14.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v13.x/api/async_hooks.html">13.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v12.x/api/async_hooks.html">12.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v11.x/api/async_hooks.html">11.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v10.x/api/async_hooks.html">10.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v9.x/api/async_hooks.html">9.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v8.x/api/async_hooks.html">8.x</a></li></ol></div>
</li>
<li class="picker-header">
<a href="#options-picker" aria-controls="options-picker">
<span class="picker-arrow"></span>
Options
</a>
<div class="picker" tabindex="-1">
<ul id="options-picker">
<li>
<a href="all.html">View on single page</a>
</li>
<li>
<a href="async_hooks.json">View as JSON</a>
</li>
<li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/async_hooks.md">Edit on GitHub</a></li>
</ul>
</div>
</li>
</ul>
</div>
<hr>
</header>
<details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><span class="stability_1"><a href="#async-hooks">Async hooks</a></span>
<ul>
<li><a href="#terminology">Terminology</a></li>
<li><a href="#overview">Overview</a></li>
<li><a href="#async_hookscreatehookcallbacks"><code>async_hooks.createHook(callbacks)</code></a>
<ul>
<li><a href="#error-handling">Error handling</a></li>
<li><a href="#printing-in-asynchook-callbacks">Printing in <code>AsyncHook</code> callbacks</a></li>
</ul>
</li>
<li><a href="#class-asynchook">Class: <code>AsyncHook</code></a>
<ul>
<li><a href="#asynchookenable"><code>asyncHook.enable()</code></a></li>
<li><a href="#asynchookdisable"><code>asyncHook.disable()</code></a></li>
<li><a href="#hook-callbacks">Hook callbacks</a>
<ul>
<li><a href="#initasyncid-type-triggerasyncid-resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a>
<ul>
<li><a href="#type"><code>type</code></a></li>
<li><a href="#triggerasyncid"><code>triggerAsyncId</code></a></li>
<li><a href="#resource"><code>resource</code></a></li>
<li><a href="#asynchronous-context-example">Asynchronous context example</a></li>
</ul>
</li>
<li><a href="#beforeasyncid"><code>before(asyncId)</code></a></li>
<li><a href="#afterasyncid"><code>after(asyncId)</code></a></li>
<li><a href="#destroyasyncid"><code>destroy(asyncId)</code></a></li>
<li><a href="#promiseresolveasyncid"><code>promiseResolve(asyncId)</code></a></li>
</ul>
</li>
<li><a href="#async_hooksexecutionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li>
<li><a href="#async_hooksexecutionasyncid"><code>async_hooks.executionAsyncId()</code></a></li>
<li><a href="#async_hookstriggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li>
<li><a href="#async_hooksasyncwrapproviders"><code>async_hooks.asyncWrapProviders</code></a></li>
</ul>
</li>
<li><a href="#promise-execution-tracking">Promise execution tracking</a></li>
<li><a href="#javascript-embedder-api">JavaScript embedder API</a>
<ul>
<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a></li>
</ul>
</li>
<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a></li>
</ul>
</li>
</ul></details>
<div role="main" id="apicontent">
<h2>Async hooks<span><a class="mark" href="#async-hooks" id="async-hooks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks"></a></h2>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental. Please migrate away from this API, if you can.
We do not recommend using the <a href="#async_hookscreatehookcallbacks"><code>createHook</code></a>, <a href="#class-asynchook"><code>AsyncHook</code></a>, and
<a href="#async_hooksexecutionasyncresource"><code>executionAsyncResource</code></a> APIs as they have usability issues, safety risks,
and performance implications. Async context tracking use cases are better
served by the stable <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> API. If you have a use case for
<code>createHook</code>, <code>AsyncHook</code>, or <code>executionAsyncResource</code> beyond the context
tracking need solved by <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> or diagnostics data currently
provided by <a href="diagnostics_channel.html">Diagnostics Channel</a>, please open an issue at
<a href="https://github.com/nodejs/node/issues">https://github.com/nodejs/node/issues</a> describing your use case so we can
create a more purpose-focused API.</div><p></p>
<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v22.14.0/lib/async_hooks.js">lib/async_hooks.js</a></p>
<p>We strongly discourage the use of the <code>async_hooks</code> API.
Other APIs that can cover most of its use cases include:</p>
<ul>
<li><a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> tracks async context</li>
<li><a href="process.html#processgetactiveresourcesinfo"><code>process.getActiveResourcesInfo()</code></a> tracks active resources</li>
</ul>
<p>The <code>node:async_hooks</code> module provides an API to track asynchronous resources.
It can be accessed using:</p>
<pre class="with-48-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> async_hooks <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);</code><button class="copy-button">copy</button></pre>
<section><h3>Terminology<span><a class="mark" href="#terminology" id="terminology">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_terminology"></a></h3>
<p>An asynchronous resource represents an object with an associated callback.
This callback may be called multiple times, such as the <code>'connection'</code>
event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>.
A resource can also be closed before the callback is called. <code>AsyncHook</code> does
not explicitly distinguish between these different cases but will represent them
as the abstract concept that is a resource.</p>
<p>If <a href="worker_threads.html#class-worker"><code>Worker</code></a>s are used, each thread has an independent <code>async_hooks</code>
interface, and each thread will use a new set of async IDs.</p>
</section><section><h3>Overview<span><a class="mark" href="#overview" id="overview">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_overview"></a></h3>
<p>Following is a simple overview of the public API.</p>
<pre class="with-48-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> async_hooks <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-comment">// Return the ID of the current execution context.</span>
<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span>
<span class="hljs-comment">// current execution scope to call.</span>
<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>();
<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span>
<span class="hljs-keyword">const</span> asyncHook =
async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve });
<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span>
<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span>
<span class="hljs-comment">// executing callbacks.</span>
asyncHook.<span class="hljs-title function_">enable</span>();
<span class="hljs-comment">// Disable listening for new asynchronous events.</span>
asyncHook.<span class="hljs-title function_">disable</span>();
<span class="hljs-comment">//</span>
<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// init() is called during object construction. The resource may not have</span>
<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span>
<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }
<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span>
<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span>
<span class="hljs-comment">// time for requests (such as FSReqCallback).</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span>
<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span>
<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-comment">// Return the ID of the current execution context.</span>
<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span>
<span class="hljs-comment">// current execution scope to call.</span>
<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>();
<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span>
<span class="hljs-keyword">const</span> asyncHook =
async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve });
<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span>
<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span>
<span class="hljs-comment">// executing callbacks.</span>
asyncHook.<span class="hljs-title function_">enable</span>();
<span class="hljs-comment">// Disable listening for new asynchronous events.</span>
asyncHook.<span class="hljs-title function_">disable</span>();
<span class="hljs-comment">//</span>
<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// init() is called during object construction. The resource may not have</span>
<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span>
<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }
<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span>
<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span>
<span class="hljs-comment">// time for requests (such as FSReqCallback).</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span>
<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span>
<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span>
<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code><button class="copy-button">copy</button></pre>
</section><section><h3><code>async_hooks.createHook(callbacks)</code><span><a class="mark" href="#async_hookscreatehookcallbacks" id="async_hookscreatehookcallbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_createhook_callbacks"></a></h3>
<div class="api_metadata">
<span>Added in: v8.1.0</span>
</div>
<ul>
<li><code>callbacks</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The <a href="#hook-callbacks">Hook Callbacks</a> to register
<ul>
<li><code>init</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#initasyncid-type-triggerasyncid-resource"><code>init</code> callback</a>.</li>
<li><code>before</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#beforeasyncid"><code>before</code> callback</a>.</li>
<li><code>after</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#afterasyncid"><code>after</code> callback</a>.</li>
<li><code>destroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#destroyasyncid"><code>destroy</code> callback</a>.</li>
<li><code>promiseResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#promiseresolveasyncid"><code>promiseResolve</code> callback</a>.</li>
</ul>
</li>
<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> Instance used for disabling and enabling hooks</li>
</ul>
<p>Registers functions to be called for different lifetime events of each async
operation.</p>
<p>The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the
respective asynchronous event during a resource's lifetime.</p>
<p>All callbacks are optional. For example, if only resource cleanup needs to
be tracked, then only the <code>destroy</code> callback needs to be passed. The
specifics of all functions that can be passed to <code>callbacks</code> is in the
<a href="#hook-callbacks">Hook Callbacks</a> section.</p>
<pre class="with-48-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createHook } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">const</span> asyncHook = <span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { },
<span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { },
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { },
<span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { },
});</code><button class="copy-button">copy</button></pre>
<p>The callbacks will be inherited via the prototype chain:</p>
<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAsyncCallbacks</span> {
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }
<span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {}
}
<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAddedCallbacks</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">MyAsyncCallbacks</span> {
<span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { }
<span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { }
}
<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">MyAddedCallbacks</span>());</code> <button class="copy-button">copy</button></pre>
<p>Because promises are asynchronous resources whose lifecycle is tracked
via the async hooks mechanism, the <code>init()</code>, <code>before()</code>, <code>after()</code>, and
<code>destroy()</code> callbacks <em>must not</em> be async functions that return promises.</p>
<h4>Error handling<span><a class="mark" href="#error-handling" id="error-handling">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_error_handling"></a></h4>
<p>If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace
and exit. The exit path does follow that of an uncaught exception, but
all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to
exit. The <code>'exit'</code> callbacks will still be called unless the application is run
with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be
printed and the application exits, leaving a core file.</p>
<p>The reason for this error handling behavior is that these callbacks are running
at potentially volatile points in an object's lifetime, for example during
class construction and destruction. Because of this, it is deemed necessary to
bring down the process quickly in order to prevent an unintentional abort in the
future. This is subject to change in the future if a comprehensive analysis is
performed to ensure an exception can follow the normal control flow without
unintentional side effects.</p>
<h4>Printing in <code>AsyncHook</code> callbacks<span><a class="mark" href="#printing-in-asynchook-callbacks" id="printing-in-asynchook-callbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_printing_in_asynchook_callbacks"></a></h4>
<p>Because printing to the console is an asynchronous operation, <code>console.log()</code>
will cause <code>AsyncHook</code> callbacks to be called. Using <code>console.log()</code> or
similar asynchronous operations inside an <code>AsyncHook</code> callback function will
cause an infinite recursion. An easy solution to this when debugging is to use a
synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>.
This will print to the file and will not invoke <code>AsyncHook</code> recursively because
it is synchronous.</p>
<pre class="with-40-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { writeFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-keyword">import</span> { format } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:util'</span>;
<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) {
<span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span>
<span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> });
}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-keyword">const</span> util = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:util'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) {
<span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span>
fs.<span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${util.format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> });
}</code><button class="copy-button">copy</button></pre>
<p>If an asynchronous operation is needed for logging, it is possible to keep
track of what caused the asynchronous operation using the information
provided by <code>AsyncHook</code> itself. The logging should then be skipped when
it was the logging itself that caused the <code>AsyncHook</code> callback to be called. By
doing this, the otherwise infinite recursion is broken.</p>
</section><section><h3>Class: <code>AsyncHook</code><span><a class="mark" href="#class-asynchook" id="class-asynchook">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_class_asynchook"></a></h3>
<p>The class <code>AsyncHook</code> exposes an interface for tracking lifetime events
of asynchronous operations.</p>
<h4><code>asyncHook.enable()</code><span><a class="mark" href="#asynchookenable" id="asynchookenable">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchook_enable"></a></h4>
<ul>
<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li>
</ul>
<p>Enable the callbacks for a given <code>AsyncHook</code> instance. If no callbacks are
provided, enabling is a no-op.</p>
<p>The <code>AsyncHook</code> instance is disabled by default. If the <code>AsyncHook</code> instance
should be enabled immediately after creation, the following pattern can be used.</p>
<pre class="with-48-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createHook } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">const</span> hook = <span class="hljs-title function_">createHook</span>(callbacks).<span class="hljs-title function_">enable</span>();</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> hook = async_hooks.<span class="hljs-title function_">createHook</span>(callbacks).<span class="hljs-title function_">enable</span>();</code><button class="copy-button">copy</button></pre>
<h4><code>asyncHook.disable()</code><span><a class="mark" href="#asynchookdisable" id="asynchookdisable">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchook_disable"></a></h4>
<ul>
<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li>
</ul>
<p>Disable the callbacks for a given <code>AsyncHook</code> instance from the global pool of
<code>AsyncHook</code> callbacks to be executed. Once a hook has been disabled it will not
be called again until enabled.</p>
<p>For API consistency <code>disable()</code> also returns the <code>AsyncHook</code> instance.</p>
<h4>Hook callbacks<span><a class="mark" href="#hook-callbacks" id="hook-callbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_hook_callbacks"></a></h4>
<p>Key events in the lifetime of asynchronous events have been categorized into
four areas: instantiation, before/after the callback is called, and when the
instance is destroyed.</p>
<h5><code>init(asyncId, type, triggerAsyncId, resource)</code><span><a class="mark" href="#initasyncid-type-triggerasyncid-resource" id="initasyncid-type-triggerasyncid-resource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_init_asyncid_type_triggerasyncid_resource"></a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> A unique ID for the async resource.</li>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The type of the async resource.</li>
<li><code>triggerAsyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The unique ID of the async resource in whose
execution context this async resource was created.</li>
<li><code>resource</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Reference to the resource representing the async
operation, needs to be released during <em>destroy</em>.</li>
</ul>
<p>Called when a class is constructed that has the <em>possibility</em> to emit an
asynchronous event. This <em>does not</em> mean the instance must call
<code>before</code>/<code>after</code> before <code>destroy</code> is called, only that the possibility
exists.</p>
<p>This behavior can be observed by doing something like opening a resource then
closing it before the resource can be used. The following snippet demonstrates
this.</p>
<pre class="with-72-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>;
<span class="hljs-title function_">createServer</span>().<span class="hljs-title function_">listen</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">close</span>(); });
<span class="hljs-comment">// OR</span>
<span class="hljs-built_in">clearTimeout</span>(<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {}, <span class="hljs-number">10</span>));</code><code class="language-js cjs"><span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>).<span class="hljs-title function_">createServer</span>().<span class="hljs-title function_">listen</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">close</span>(); });
<span class="hljs-comment">// OR</span>
<span class="hljs-built_in">clearTimeout</span>(<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {}, <span class="hljs-number">10</span>));</code><button class="copy-button">copy</button></pre>
<p>Every new resource is assigned an ID that is unique within the scope of the
current Node.js instance.</p>
<h6><code>type</code><span><a class="mark" href="#type" id="type">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_type"></a></h6>
<p>The <code>type</code> is a string identifying the type of resource that caused
<code>init</code> to be called. Generally, it will correspond to the name of the
resource's constructor.</p>
<p>The <code>type</code> of resources created by Node.js itself can change in any Node.js
release. Valid values include <code>TLSWRAP</code>,
<code>TCPWRAP</code>, <code>TCPSERVERWRAP</code>, <code>GETADDRINFOREQWRAP</code>, <code>FSREQCALLBACK</code>,
<code>Microtask</code>, and <code>Timeout</code>. Inspect the source code of the Node.js version used
to get the full list.</p>
<p>Furthermore users of <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a> create async resources independent
of Node.js itself.</p>
<p>There is also the <code>PROMISE</code> resource type, which is used to track <code>Promise</code>
instances and asynchronous work scheduled by them.</p>
<p>Users are able to define their own <code>type</code> when using the public embedder API.</p>
<p>It is possible to have type name collisions. Embedders are encouraged to use
unique prefixes, such as the npm package name, to prevent collisions when
listening to the hooks.</p>
<h6><code>triggerAsyncId</code><span><a class="mark" href="#triggerasyncid" id="triggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_triggerasyncid"></a></h6>
<p><code>triggerAsyncId</code> is the <code>asyncId</code> of the resource that caused (or "triggered")
the new resource to initialize and that caused <code>init</code> to call. This is different
from <code>async_hooks.executionAsyncId()</code> that only shows <em>when</em> a resource was
created, while <code>triggerAsyncId</code> shows <em>why</em> a resource was created.</p>
<p>The following is a simple demonstration of <code>triggerAsyncId</code>:</p>
<pre class="with-69-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createHook, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>;
<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>;
<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) {
<span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>();
fs.<span class="hljs-title function_">writeSync</span>(
stdout.<span class="hljs-property">fd</span>,
<span class="hljs-string">`<span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>): trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>);
},
}).<span class="hljs-title function_">enable</span>();
net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createHook, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> { stdout } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:process'</span>);
<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>);
<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) {
<span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>();
fs.<span class="hljs-title function_">writeSync</span>(
stdout.<span class="hljs-property">fd</span>,
<span class="hljs-string">`<span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>): trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>);
},
}).<span class="hljs-title function_">enable</span>();
net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);</code><button class="copy-button">copy</button></pre>
<p>Output when hitting the server with <code>nc localhost 8080</code>:</p>
<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1
TCPWRAP(7): trigger: 5 execution: 0</code> <button class="copy-button">copy</button></pre>
<p>The <code>TCPSERVERWRAP</code> is the server which receives the connections.</p>
<p>The <code>TCPWRAP</code> is the new connection from the client. When a new
connection is made, the <code>TCPWrap</code> instance is immediately constructed. This
happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means
that it is being executed from C++ with no JavaScript stack above it.) With only
that information, it would be impossible to link resources together in
terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task
of propagating what resource is responsible for the new resource's existence.</p>
<h6><code>resource</code><span><a class="mark" href="#resource" id="resource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_resource"></a></h6>
<p><code>resource</code> is an object that represents the actual async resource that has
been initialized. The API to access the object may be specified by the
creator of the resource. Resources created by Node.js itself are internal
and may change at any time. Therefore no API is specified for these.</p>
<p>In some cases the resource object is reused for performance reasons, it is
thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.</p>
<h6>Asynchronous context example<span><a class="mark" href="#asynchronous-context-example" id="asynchronous-context-example">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchronous_context_example"></a></h6>
<p>The context tracking use case is covered by the stable API <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>.
This example only illustrates async hooks operation but <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>
fits better to this use case.</p>
<p>The following is an example with additional information about the calls to
<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the
callback to <code>listen()</code> will look like. The output formatting is slightly more
elaborate to make calling context easier to see.</p>
<pre class="with-48-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> async_hooks <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>;
<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>;
<span class="hljs-keyword">const</span> { fd } = stdout;
<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>;
async_hooks.<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) {
<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(
fd,
<span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> +
<span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>);
},
<span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) {
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>before: <span class="hljs-subst">${asyncId}</span>\n`</span>);
indent += <span class="hljs-number">2</span>;
},
<span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) {
indent -= <span class="hljs-number">2</span>;
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>after: <span class="hljs-subst">${asyncId}</span>\n`</span>);
},
<span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>destroy: <span class="hljs-subst">${asyncId}</span>\n`</span>);
},
}).<span class="hljs-title function_">enable</span>();
net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">() =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>, <span class="hljs-function">() =></span> {
<span class="hljs-comment">// Let's wait 10ms before logging the server started.</span>
<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'>>>'</span>, async_hooks.<span class="hljs-title function_">executionAsyncId</span>());
}, <span class="hljs-number">10</span>);
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>);
<span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>;
<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>;
async_hooks.<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) {
<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(
fd,
<span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> +
<span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>);
},
<span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) {
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>before: <span class="hljs-subst">${asyncId}</span>\n`</span>);
indent += <span class="hljs-number">2</span>;
},
<span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) {
indent -= <span class="hljs-number">2</span>;
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>after: <span class="hljs-subst">${asyncId}</span>\n`</span>);
},
<span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {
<span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent);
fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>destroy: <span class="hljs-subst">${asyncId}</span>\n`</span>);
},
}).<span class="hljs-title function_">enable</span>();
net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">() =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>, <span class="hljs-function">() =></span> {
<span class="hljs-comment">// Let's wait 10ms before logging the server started.</span>
<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'>>>'</span>, async_hooks.<span class="hljs-title function_">executionAsyncId</span>());
}, <span class="hljs-number">10</span>);
});</code><button class="copy-button">copy</button></pre>
<p>Output from only starting the server:</p>
<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before: 6
Timeout(7): trigger: 6 execution: 6
after: 6
destroy: 6
before: 7
<span class="hljs-meta prompt_">></span><span class="language-bash">>> 7</span>
TickObject(8): trigger: 7 execution: 7
after: 7
before: 8
after: 8</code> <button class="copy-button">copy</button></pre>
<p>As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify
the value of the current execution context; which is delineated by calls to
<code>before</code> and <code>after</code>.</p>
<p>Only using <code>execution</code> to graph resource allocation results in the following:</p>
<pre><code class="language-console"> root(1)
^
|
TickObject(6)
^
|
Timeout(7)</code> <button class="copy-button">copy</button></pre>
<p>The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for
<code>console.log()</code> being called. This is because binding to a port without a host
name is a <em>synchronous</em> operation, but to maintain a completely asynchronous
API the user's callback is placed in a <code>process.nextTick()</code>. Which is why
<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code>
callback.</p>
<p>The graph only shows <em>when</em> a resource was created, not <em>why</em>, so to track
the <em>why</em> use <code>triggerAsyncId</code>. Which can be represented with the following
graph:</p>
<pre><code class="language-console"> bootstrap(1)
|
˅
TCPSERVERWRAP(5)
|
˅
TickObject(6)
|
˅
Timeout(7)</code> <button class="copy-button">copy</button></pre>
<h5><code>before(asyncId)</code><span><a class="mark" href="#beforeasyncid" id="beforeasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_before_asyncid"></a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
<p>When an asynchronous operation is initiated (such as a TCP server receiving a
new connection) or completes (such as writing data to disk) a callback is
called to notify the user. The <code>before</code> callback is called just before said
callback is executed. <code>asyncId</code> is the unique identifier assigned to the
resource about to execute the callback.</p>
<p>The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback
will typically be called 0 times if the asynchronous operation was cancelled
or, for example, if no connections are received by a TCP server. Persistent
asynchronous resources like a TCP server will typically call the <code>before</code>
callback multiple times, while other operations like <code>fs.open()</code> will call
it only once.</p>
<h5><code>after(asyncId)</code><span><a class="mark" href="#afterasyncid" id="afterasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_after_asyncid"></a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
<p>Called immediately after the callback specified in <code>before</code> is completed.</p>
<p>If an uncaught exception occurs during execution of the callback, then <code>after</code>
will run <em>after</em> the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s
handler runs.</p>
<h5><code>destroy(asyncId)</code><span><a class="mark" href="#destroyasyncid" id="destroyasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_destroy_asyncid"></a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
<p>Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also
called asynchronously from the embedder API <code>emitDestroy()</code>.</p>
<p>Some resources depend on garbage collection for cleanup, so if a reference is
made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code>
will never be called, causing a memory leak in the application. If the resource
does not depend on garbage collection, then this will not be an issue.</p>
<p>Using the destroy hook results in additional overhead because it enables
tracking of <code>Promise</code> instances via the garbage collector.</p>
<h5><code>promiseResolve(asyncId)</code><span><a class="mark" href="#promiseresolveasyncid" id="promiseresolveasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_promiseresolve_asyncid"></a></h5>
<div class="api_metadata">
<span>Added in: v8.6.0</span>
</div>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
<p>Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is
invoked (either directly or through other means of resolving a promise).</p>
<p><code>resolve()</code> does not do any observable synchronous work.</p>
<p>The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the
<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.</p>
<pre><code class="language-js"><span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve</span>) =></span> <span class="hljs-title function_">resolve</span>(<span class="hljs-literal">true</span>)).<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">a</span>) =></span> {});</code> <button class="copy-button">copy</button></pre>
<p>calls the following callbacks:</p>
<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1
promise resolve 5 # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5 # the Promise returned by then()
before 6 # the then() callback is entered
promise resolve 6 # the then() callback resolves the promise by returning
after 6</code> <button class="copy-button">copy</button></pre>
<h4><code>async_hooks.executionAsyncResource()</code><span><a class="mark" href="#async_hooksexecutionasyncresource" id="async_hooksexecutionasyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_executionasyncresource"></a></h4>
<div class="api_metadata">
<span>Added in: v13.9.0, v12.17.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The resource representing the current execution.
Useful to store data within the resource.</li>
</ul>
<p>Resource objects returned by <code>executionAsyncResource()</code> are most often internal
Node.js handle objects with undocumented APIs. Using any functions or properties
on the object is likely to crash your application and should be avoided.</p>
<p>Using <code>executionAsyncResource()</code> in the top-level execution context will
return an empty object as there is no handle or request object to use,
but having an object representing the top-level can be helpful.</p>
<pre class="with-81-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { open } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-keyword">import</span> { executionAsyncId, executionAsyncResource } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 1 {}</span>
<span class="hljs-title function_">open</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>), <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 7 FSReqWrap</span>
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { open } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-keyword">const</span> { executionAsyncId, executionAsyncResource } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 1 {}</span>
<span class="hljs-title function_">open</span>(__filename, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 7 FSReqWrap</span>
});</code><button class="copy-button">copy</button></pre>
<p>This can be used to implement continuation local storage without the
use of a tracking <code>Map</code> to store the metadata:</p>
<pre class="with-46-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>;
<span class="hljs-keyword">import</span> {
executionAsyncId,
executionAsyncResource,
createHook,
} <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">const</span> sym = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'state'</span>); <span class="hljs-comment">// Private symbol to avoid pollution</span>
<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) {
<span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>();
<span class="hljs-keyword">if</span> (cr) {
resource[sym] = cr[sym];
}
},
}).<span class="hljs-title function_">enable</span>();
<span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
<span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> };
<span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) {
res.<span class="hljs-title function_">end</span>(<span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">stringify</span>(<span class="hljs-title function_">executionAsyncResource</span>()[sym]));
}, <span class="hljs-number">100</span>);
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createServer } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>);
<span class="hljs-keyword">const</span> {
executionAsyncId,
executionAsyncResource,
createHook,
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> sym = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'state'</span>); <span class="hljs-comment">// Private symbol to avoid pollution</span>
<span class="hljs-title function_">createHook</span>({
<span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) {
<span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>();
<span class="hljs-keyword">if</span> (cr) {
resource[sym] = cr[sym];
}
},
}).<span class="hljs-title function_">enable</span>();
<span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
<span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> };
<span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) {
res.<span class="hljs-title function_">end</span>(<span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">stringify</span>(<span class="hljs-title function_">executionAsyncResource</span>()[sym]));
}, <span class="hljs-number">100</span>);
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><button class="copy-button">copy</button></pre>
<h4><code>async_hooks.executionAsyncId()</code><span><a class="mark" href="#async_hooksexecutionasyncid" id="async_hooksexecutionasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_executionasyncid"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v8.2.0</td>
<td><p>Renamed from <code>currentId</code>.</p></td></tr>
<tr><td>v8.1.0</td>
<td><p><span>Added in: v8.1.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The <code>asyncId</code> of the current execution context. Useful to
track when something calls.</li>
</ul>
<pre class="with-52-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>;
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 1 - bootstrap</span>
<span class="hljs-keyword">const</span> path = <span class="hljs-string">'.'</span>;
fs.<span class="hljs-title function_">open</span>(path, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 6 - open()</span>
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 1 - bootstrap</span>
<span class="hljs-keyword">const</span> path = <span class="hljs-string">'.'</span>;
fs.<span class="hljs-title function_">open</span>(path, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 6 - open()</span>
});</code><button class="copy-button">copy</button></pre>
<p>The ID returned from <code>executionAsyncId()</code> is related to execution timing, not
causality (which is covered by <code>triggerAsyncId()</code>):</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> server = net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {
<span class="hljs-comment">// Returns the ID of the server, not of the new connection, because the</span>
<span class="hljs-comment">// callback runs in the execution scope of the server's MakeCallback().</span>
async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> {
<span class="hljs-comment">// Returns the ID of a TickObject (process.nextTick()) because all</span>
<span class="hljs-comment">// callbacks passed to .listen() are wrapped in a nextTick().</span>
async_hooks.<span class="hljs-title function_">executionAsyncId</span>();
});</code> <button class="copy-button">copy</button></pre>
<p>Promise contexts may not get precise <code>executionAsyncIds</code> by default.
See the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p>
<h4><code>async_hooks.triggerAsyncId()</code><span><a class="mark" href="#async_hookstriggerasyncid" id="async_hookstriggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_triggerasyncid"></a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The ID of the resource responsible for calling the callback
that is currently being executed.</li>
</ul>
<pre><code class="language-js"><span class="hljs-keyword">const</span> server = net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {
<span class="hljs-comment">// The resource that caused (or triggered) this callback to be called</span>
<span class="hljs-comment">// was that of the new connection. Thus the return value of triggerAsyncId()</span>
<span class="hljs-comment">// is the asyncId of "conn".</span>
async_hooks.<span class="hljs-title function_">triggerAsyncId</span>();
}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> {
<span class="hljs-comment">// Even though all callbacks passed to .listen() are wrapped in a nextTick()</span>
<span class="hljs-comment">// the callback itself exists because the call to the server's .listen()</span>
<span class="hljs-comment">// was made. So the return value would be the ID of the server.</span>
async_hooks.<span class="hljs-title function_">triggerAsyncId</span>();
});</code> <button class="copy-button">copy</button></pre>
<p>Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See
the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p>
<h4><code>async_hooks.asyncWrapProviders</code><span><a class="mark" href="#async_hooksasyncwrapproviders" id="async_hooksasyncwrapproviders">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_asyncwrapproviders"></a></h4>
<div class="api_metadata">
<span>Added in: v17.2.0, v16.14.0</span>
</div>
<ul>
<li>Returns: A map of provider types to the corresponding numeric id.
This map contains all the event types that might be emitted by the <code>async_hooks.init()</code> event.</li>
</ul>
<p>This feature suppresses the deprecated usage of <code>process.binding('async_wrap').Providers</code>.
See: <a href="deprecations.html#dep0111-processbinding">DEP0111</a></p>
</section><section><h3>Promise execution tracking<span><a class="mark" href="#promise-execution-tracking" id="promise-execution-tracking">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_promise_execution_tracking"></a></h3>
<p>By default, promise executions are not assigned <code>asyncId</code>s due to the relatively
expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by
V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get
correct execution and trigger ids for promise callback contexts by default.</p>
<pre class="with-73-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { executionAsyncId, triggerAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>);
});
<span class="hljs-comment">// produces:</span>
<span class="hljs-comment">// eid 1 tid 0</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> { executionAsyncId, triggerAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>);
});
<span class="hljs-comment">// produces:</span>
<span class="hljs-comment">// eid 1 tid 0</span></code><button class="copy-button">copy</button></pre>
<p>Observe that the <code>then()</code> callback claims to have executed in the context of the
outer scope even though there was an asynchronous hop involved. Also,
the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about
the resource that caused (triggered) the <code>then()</code> callback to be executed.</p>
<p>Installing async hooks via <code>async_hooks.createHook</code> enables promise execution
tracking:</p>
<pre class="with-85-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createHook, executionAsyncId, triggerAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-title function_">createHook</span>({ <span class="hljs-title function_">init</span>(<span class="hljs-params"></span>) {} }).<span class="hljs-title function_">enable</span>(); <span class="hljs-comment">// forces PromiseHooks to be enabled.</span>
<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>);
});
<span class="hljs-comment">// produces:</span>
<span class="hljs-comment">// eid 7 tid 6</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createHook, executionAsyncId, triggerAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-title function_">createHook</span>({ <span class="hljs-title function_">init</span>(<span class="hljs-params"></span>) {} }).<span class="hljs-title function_">enable</span>(); <span class="hljs-comment">// forces PromiseHooks to be enabled.</span>
<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>);
});
<span class="hljs-comment">// produces:</span>
<span class="hljs-comment">// eid 7 tid 6</span></code><button class="copy-button">copy</button></pre>
<p>In this example, adding any actual hook function enabled the tracking of
promises. There are two promises in the example above; the promise created by
<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the
example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got
<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing
in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by
async resource <code>6</code>.</p>
<p>Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run
only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code>
will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details
see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.</p>
</section><section><h3>JavaScript embedder API<span><a class="mark" href="#javascript-embedder-api" id="javascript-embedder-api">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_javascript_embedder_api"></a></h3>
<p>Library developers that handle their own asynchronous resources performing tasks
like I/O, connection pooling, or managing callback queues may use the
<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.</p>
<h4>Class: <code>AsyncResource</code><span><a class="mark" href="#class-asyncresource" id="class-asyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_class_asyncresource"></a></h4>
<p>The documentation for this class has moved <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a>.</p>
</section><section><h3>Class: <code>AsyncLocalStorage</code><span><a class="mark" href="#class-asynclocalstorage" id="class-asynclocalstorage">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_class_asynclocalstorage"></a></h3>
<p>The documentation for this class has moved <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>.</p></section>
<!-- API END -->
</div>
</div>
</div>
</body>
</html>
|
