Kindof.js
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 literal ("Hello"
), a boxed object (new String("Hello")
) 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 want to handle both literal primitives
42
and objects new Number(42)
(for robustness) the same way.
A single kindof(num) == "number"
check makes that easy.
Be sure to compare with ==
to allow for coercion in that case. - 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 supports all ECMAScript built-in types and classes: undefined
, null
, Boolean
, Number
, String
, 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 (numbers, dates etc.) or proper arrays have a kind other than object
.
Please see the table below for the full list of kinds.
Installing
Note: Kindof.js follows semantic versioning.
Installing for the browser
Take the kindof.js
file and source it at will.
Installing on Node
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:
kindof("Hello")
kindof(new String("Hello"))
A switch statement might help:
switch (kindof(obj)) {
case "null": this.name = "Alfred"; break
case "string": this.name = obj; break
case "number": this.age = obj; break
default: throw new TypeError("Pardon, sir, came upon an unexpected type.")
}
Kinds
The pattern is simple — all built-in objects that behave like value objects (numbers, strings, dates etc.) or are real arrays are said to be of a kind other than object
. The arguments
object, however, is not a proper array, so it therefore is an object
.
Value | Kindof |
---|
undefined | undefined |
null | null |
true | boolean |
false | boolean |
new Boolean(true) | boolean |
42 | number |
new Number(42) | number |
NaN | number |
Infinity | number |
"Hello" | string |
new String("Hello") | string |
/.*/ | regexp |
new RegExp(".*") | regexp |
new Date | date |
[42, 69] | array |
function() {} | function |
{} | object |
arguments | 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
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 or create an issue online.