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
|
Kindof.js
=========
[![NPM version][npm-badge]](https://www.npmjs.com/package/kindof)
[![Build status][travis-badge]](https://travis-ci.org/moll/js-kindof)
Kindof.js **provides a single `kindof` function** that does what you'd expect
from `typeof` — gives you the proper semantic type regardless if the variable
was a **primitive** (`"Hello"`), a **built-in [value object][value-object]**
like (`new Date(2000, 5, 18)` or `/.*/`) or came from **another execution
context** (e.g. an array from another `<iframe>`).
### Tour
When and why should you use `kindof` over `typeof`?
- When you need a type check that returns `"null"` given the `null` value.
You might remember, JavaScript's `typeof` says `null` is an object.
- When you need to **differentiate** between **plain objects** (`{name:
"John"}`) and **built-in value objects** (`new Date(2000, 5, 18)`).
A single `kindof(obj) == "date"` check makes that easy.
- When there's a chance you might get an object from **another execution
context**.
In the browser that might mean an object from another `<frame>`.
Different execution contexts have different built-in class instances, so you
can't do `obj instanceof Date` safely.
- Kindof.js does not consider boxed objects (instances of `Boolean`, `Number`
and `String`) to of the same type as their primitive counterparts. See below
for why boxed objects are very error prone and should be avoided.
Kindof.js supports all ECMAScript **built-in types and primitives**:
`undefined`, `null`, `Boolean`, `Number`, `String`, `Symbol`, `RegExp`, `Date`,
`Array`, `Function` and plain old `Object`. Others, e.g. `Math` and `JSON`, are
considered just objects. In general, objects that behave like **value objects**
(dates, regular expressions etc.) or proper arrays have a kind other than
`object`.
Please see the table below for the full list of kinds.
### Primitives and Boxed Objects
You might know, JavaScript has both primitive types and boxed object types for
booleans, numbers and strings. Primitives are what you get from code literals
(`true`, `42`, `"Hello"`) and from `JSON.parse`. Boxed objects tend to only
appear when someone explicitly calls their constructor (`new Boolean(false)`).
Boxed objects wouldn't be so bad, except JavaScript's equivalence operator
(`==`), for all its type coercions, doesn't handle them transparently. While you
can't compare other value types like dates and regular expressions with `==`
either, you won't make that mistake that easily. The following is a small
example of problems with boxed objects:
```javascript
new String("a") == new String("a") // => false
new Boolean(true) == new Boolean(true) // => false
Boolean(new Boolean(false)) // => true
!!(new Boolean(false)) // => true
```
If you still wish Kindof to consider boxed Boolean, Number and String types like
primitives (returning `"boolean"`, `"number"` and `"string"` respectively), feel
free to use [Kindof.js's v1 branch][v1] with `npm install kindof@1`.
[value-object]: https://en.wikipedia.org/wiki/Value_object
[v1]: https://github.com/moll/js-kindof/tree/v1
[npm-badge]: https://img.shields.io/npm/v/kindof.svg
[travis-badge]: https://travis-ci.org/moll/js-kindof.png?branch=master
Installing
----------
**Note**: Kindof.js follows [semantic versioning](http://semver.org/).
### Installing for the browser
Take the `kindof.js` file and source it at will.
### Installing on Node.js
Install with `npm install kindof`.
And require with `var kindof = require("kindof")`.
Using
-----
Pass any object to `kindof` and compare its output to what you expect:
```javascript
kindof("Hello") // => "string"
kindof(new Date(2000, 5, 18)) // => "date"
```
A switch statement might help:
```javascript
switch (kindof(obj)) {
case "null": this.name = "Alfred"; break
case "string": this.name = obj; break
case "date": this.birthdate = obj; break
default: throw new TypeError("Pardon, sir, came upon an unexpected type.")
}
```
Kinds
-----
The pattern is simple and follows `typeof`: besides primitives, built-in objects
that are **value objects** (dates, regular expressions etc.) or **real arrays**
are of a kind other than `object`. The `arguments` object, for example, is not
a proper array and is therefore an `object`.
Value | Kindof
----------------------|----------
`undefined `| undefined
`null `| null
`true `| boolean
`false `| boolean
`42 `| number
`NaN `| number
`Infinity `| number
`"Hello" `| string
`Symbol() `| symbol
`Symbol("forEach") `| symbol
`Symbol.iterator `| symbol
`/.*/ `| regexp
`new RegExp(".*") `| regexp
`new Date `| date
`[42, 69] `| array
`function() {} `| function
`{} `| object
`arguments `| object
`new Boolean(true) `| object
`new Number(42) `| object
`new String("Hello") `| object
`new MyClass `| object
`new Error `| object
`Math `| object
`JSON `| object
**Subclassed objects**, such as subclassed arrays, are considered to be `object`
unless their internal `[[Class]]` property remains that of the original. For
ways to subclass properly, please see further reading below.
Further Reading
---------------
- The [`typeof` operator in ECMAScript 5.1][typeof-ecma].
- The [`typeof` operator as implemented by Firefox][typeof-firefox].
- Article on [subclassing `Array`][subclass] by [Juriy Zaytsev][juriy].
[typeof-ecma]: http://www.ecma-international.org/ecma-262/5.1/#sec-11.4.3
[typeof-firefox]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof
[subclass]: http://perfectionkills.com/how-ecmascript-5-still-does-not-allow-to-subclass-an-array/
[juriy]: http://perfectionkills.com
License
-------
Kindof.js is released under a *Lesser GNU Affero General Public License*, which
in summary means:
- You **can** use this program for **no cost**.
- You **can** use this program for **both personal and commercial reasons**.
- You **do not have to share your own program's code** which uses this program.
- You **have to share modifications** (e.g bug-fixes) you've made to this
program.
For more convoluted language, see the `LICENSE` file.
About
-----
**[Andri Möll](http://themoll.com)** typed this and the code.
[Monday Calendar](https://mondayapp.com) supported the engineering work.
If you find Kindof.js needs improving, please don't hesitate to type to me now
at [andri@dot.ee][email] or [create an issue online][issues].
[email]: mailto:andri@dot.ee
[issues]: https://github.com/moll/js-kindof/issues
|
