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 ("Hello"
), a built-in 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 thenull
value.
You might remember, JavaScript'stypeof
saysnull
is an object. - When you need to differentiate between plain objects (
{name: "John"}
) and built-in value objects (new Date(2000, 5, 18)
).
A singlekindof(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 doobj instanceof Date
safely. - Kindof.js does not consider boxed objects (instances of
Boolean
,Number
andString
) 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:
"a" == "a" // => falsetrue == true // => falseBooleanfalse // => true!!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 with npm install kindof@1
.
Installing
Note: Kindof.js follows semantic versioning.
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:
// => "string" // => "date"
A switch statement might help:
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. - The
typeof
operator as implemented by Firefox. - Article on subclassing
Array
by Juriy Zaytsev.
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.