A proper typeof that works with primitives, built-in value objects and those from other execution contexts.
npm install kindofKindof.js
=========
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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 ).
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:) and built-in value objects (
"John"}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 .
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 thanobject.
Please see the table below for the full list of kinds.
true, 42, "Hello") and from JSON.parse. Boxed objects tend to onlynew 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), feelnpm install kindof@1
free to use [Kindof.js's v1 branch][v1] with .
[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.
file and source it at will.$3
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 | objectSubclassed 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 typed this and the code.
Monday Calendar 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