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<ul>
<li><span class="stability_2"><a href="#console_console">Console</a></span><ul>
<li><span class="stability_undefined"><a href="#console_asynchronous_vs_synchronous_consoles">Asynchronous vs Synchronous Consoles</a></span></li>
<li><span class="stability_undefined"><a href="#console_class_console">Class: Console</a></span><ul>
<li><span class="stability_undefined"><a href="#console_new_console_stdout_stderr">new Console(stdout[, stderr])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_assert_value_message">console.assert(value[, message][, ...])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_dir_obj_options">console.dir(obj[, options])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_error_data">console.error([data][, ...])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_info_data">console.info([data][, ...])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_log_data">console.log([data][, ...])</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_time_label">console.time(label)</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_timeend_label">console.timeEnd(label)</a></span></li>
<li><span class="stability_undefined"><a href="#console_console_trace_message">console.trace(message[, ...])</a></span></li>
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<div id="apicontent">
<h1>Console<span><a class="mark" href="#console_console" id="console_console">#</a></span></h1>
<pre class="api_stability api_stability_2">Stability: 2 - Stable</pre><p>The <code>console</code> module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.</p>
<p>The module exports two specific components:</p>
<ul>
<li>A <code>Console</code> class with methods such as <code>console.log()</code>, <code>console.error()</code> and
<code>console.warn()</code> that can be used to write to any Node.js stream.</li>
<li>A global <code>console</code> instance configured to write to <code>stdout</code> and <code>stderr</code>.
Because this object is global, it can be used without calling
<code>require('console')</code>.</li>
</ul>
<p>Example using the global <code>console</code>:</p>
<pre><code class="lang-js">console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr
const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr
</code></pre>
<p>Example using the <code>Console</code> class:</p>
<pre><code class="lang-js">const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);
myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err
const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
</code></pre>
<p>While the API for the <code>Console</code> class is designed fundamentally around the
browser <code>console</code> object, the <code>Console</code> in Node.js is <em>not</em> intended to
duplicate the browser's functionality exactly.</p>
<h2>Asynchronous vs Synchronous Consoles<span><a class="mark" href="#console_asynchronous_vs_synchronous_consoles" id="console_asynchronous_vs_synchronous_consoles">#</a></span></h2>
<p>The console functions are usually asynchronous unless the destination is a file.
Disks are fast and operating systems normally employ write-back caching;
it should be a very rare occurrence indeed that a write blocks, but it
is possible.</p>
<p>Additionally, console functions are blocking when outputting to TTYs
(terminals) on OS X as a workaround for the OS's very small, 1kb buffer size.
This is to prevent interleaving between <code>stdout</code> and <code>stderr</code>.</p>
<h2>Class: Console<span><a class="mark" href="#console_class_console" id="console_class_console">#</a></span></h2>
<!--type=class-->
<p>The <code>Console</code> class can be used to create a simple logger with configurable
output streams and can be accessed using either <code>require('console').Console</code>
or <code>console.Console</code>:</p>
<pre><code class="lang-js">const Console = require('console').Console;
const Console = console.Console;
</code></pre>
<h3>new Console(stdout[, stderr])<span><a class="mark" href="#console_new_console_stdout_stderr" id="console_new_console_stdout_stderr">#</a></span></h3>
<p>Creates a new <code>Console</code> by passing one or two writable stream instances.
<code>stdout</code> is a writable stream to print log or info output. <code>stderr</code>
is used for warning or error output. If <code>stderr</code> isn't passed, warning and error
output will be sent to <code>stdout</code>.</p>
<pre><code class="lang-js">const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
const count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5
</code></pre>
<p>The global <code>console</code> is a special <code>Console</code> whose output is sent to
<a href="process.html#process_process_stdout"><code>process.stdout</code></a> and <a href="process.html#process_process_stderr"><code>process.stderr</code></a>. It is equivalent to calling:</p>
<pre><code class="lang-js">new Console(process.stdout, process.stderr);
</code></pre>
<h3>console.assert(value[, message][, ...])<span><a class="mark" href="#console_console_assert_value_message" id="console_console_assert_value_message">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.101</span>
</div><p>A simple assertion test that verifies whether <code>value</code> is truthy. If it is not,
an <code>AssertionError</code> is thrown. If provided, the error <code>message</code> is formatted
using <a href="util.html#util_util_format_format"><code>util.format()</code></a> and used as the error message.</p>
<pre><code class="lang-js">console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work
</code></pre>
<p><em>Note: the <code>console.assert()</code> method is implemented differently in Node.js
than the <code>console.assert()</code> method <a href="https://developer.mozilla.org/en-US/docs/Web/API/console/assert">available in browsers</a>.</em></p>
<p>Specifically, in browsers, calling <code>console.assert()</code> with a falsy
assertion will cause the <code>message</code> to be printed to the console without
interrupting execution of subsequent code. In Node.js, however, a falsy
assertion will cause an <code>AssertionError</code> to be thrown.</p>
<p>Functionality approximating that implemented by browsers can be implemented
by extending Node.js' <code>console</code> and overriding the <code>console.assert()</code> method.</p>
<p>In the following example, a simple module is created that extends and overrides
the default behavior of <code>console</code> in Node.js.</p>
<pre><code class="lang-js">'use strict';
// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.create(console, {
assert: {
value: function assert(assertion, message, ...args) {
try {
console.assert(assertion, message, ...args);
} catch (err) {
console.error(err.stack);
}
},
configurable: true,
enumerable: true,
writable: true,
},
});
module.exports = myConsole;
</code></pre>
<p>This can then be used as a direct replacement for the built in console:</p>
<pre><code class="lang-js">const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');
</code></pre>
<h3>console.dir(obj[, options])<span><a class="mark" href="#console_console_dir_obj_options" id="console_console_dir_obj_options">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.101</span>
</div><p>Uses <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> on <code>obj</code> and prints the resulting string to <code>stdout</code>.
This function bypasses any custom <code>inspect()</code> function defined on <code>obj</code>. An
optional <code>options</code> object may be passed to alter certain aspects of the
formatted string:</p>
<ul>
<li><p><code>showHidden</code> - if <code>true</code> then the object's non-enumerable and symbol
properties will be shown too. Defaults to <code>false</code>.</p>
</li>
<li><p><code>depth</code> - tells <a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> how many times to recurse while
formatting the object. This is useful for inspecting large complicated objects.
Defaults to <code>2</code>. To make it recurse indefinitely, pass <code>null</code>.</p>
</li>
<li><p><code>colors</code> - if <code>true</code>, then the output will be styled with ANSI color codes.
Defaults to <code>false</code>. Colors are customizable; see
<a href="util.html#util_customizing_util_inspect_colors">customizing <code>util.inspect()</code> colors</a>.</p>
</li>
</ul>
<h3>console.error([data][, ...])<span><a class="mark" href="#console_console_error_data" id="console_console_error_data">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.100</span>
</div><p>Prints to <code>stderr</code> with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to <a href="http://man7.org/linux/man-pages/man3/printf.3.html">printf(3)</a> (the arguments are all passed to
<a href="util.html#util_util_format_format"><code>util.format()</code></a>).</p>
<pre><code class="lang-js">const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr
</code></pre>
<p>If formatting elements (e.g. <code>%d</code>) are not found in the first string then
<a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> is called on each argument and the resulting string
values are concatenated. See <a href="util.html#util_util_format_format"><code>util.format()</code></a> for more information.</p>
<h3>console.info([data][, ...])<span><a class="mark" href="#console_console_info_data" id="console_console_info_data">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.100</span>
</div><p>The <code>console.info()</code> function is an alias for <a href="#console_console_log_data"><code>console.log()</code></a>.</p>
<h3>console.log([data][, ...])<span><a class="mark" href="#console_console_log_data" id="console_console_log_data">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.100</span>
</div><p>Prints to <code>stdout</code> with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to <a href="http://man7.org/linux/man-pages/man3/printf.3.html">printf(3)</a> (the arguments are all passed to
<a href="util.html#util_util_format_format"><code>util.format()</code></a>).</p>
<pre><code class="lang-js">const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout
</code></pre>
<p>If formatting elements (e.g. <code>%d</code>) are not found in the first string then
<a href="util.html#util_util_inspect_object_options"><code>util.inspect()</code></a> is called on each argument and the resulting string
values are concatenated. See <a href="util.html#util_util_format_format"><code>util.format()</code></a> for more information.</p>
<h3>console.time(label)<span><a class="mark" href="#console_console_time_label" id="console_console_time_label">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.104</span>
</div><p>Used to calculate the duration of a specific operation. To start a timer, call
the <code>console.time()</code> method, giving it a unique <code>label</code> as the only parameter. To stop the
timer, and to get the elapsed time in milliseconds, just call the
<a href="#console_console_timeend_label"><code>console.timeEnd()</code></a> method, again passing the
timer's unique <code>label</code> as the parameter.</p>
<h3>console.timeEnd(label)<span><a class="mark" href="#console_console_timeend_label" id="console_console_timeend_label">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.104</span>
</div><p>Stops a timer that was previously started by calling <a href="#console_console_time_label"><code>console.time()</code></a> and
prints the result to stdout:</p>
<pre><code class="lang-js">console.time('100-elements');
for (let i = 0; i < 100; i++) {
;
}
console.timeEnd('100-elements');
// prints 100-elements: 262ms
</code></pre>
<h3>console.trace(message[, ...])<span><a class="mark" href="#console_console_trace_message" id="console_console_trace_message">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.104</span>
</div><p>Prints to <code>stderr</code> the string <code>'Trace :'</code>, followed by the <a href="util.html#util_util_format_format"><code>util.format()</code></a>
formatted message and stack trace to the current position in the code.</p>
<pre><code class="lang-js">console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
// Trace: Show me
// at repl:2:9
// at REPLServer.defaultEval (repl.js:248:27)
// at bound (domain.js:287:14)
// at REPLServer.runBound [as eval] (domain.js:300:12)
// at REPLServer.<anonymous> (repl.js:412:12)
// at emitOne (events.js:82:20)
// at REPLServer.emit (events.js:169:7)
// at REPLServer.Interface._onLine (readline.js:210:10)
// at REPLServer.Interface._line (readline.js:549:8)
// at REPLServer.Interface._ttyWrite (readline.js:826:14)
</code></pre>
<h3>console.warn([data][, ...])<span><a class="mark" href="#console_console_warn_data" id="console_console_warn_data">#</a></span></h3>
<div class="api_metadata">
<span>Added in: v0.1.100</span>
</div><p>The <code>console.warn()</code> function is an alias for <a href="#console_console_error_data"><code>console.error()</code></a>.</p>
</div>
</div>
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