kindof

A proper typeof that works with primitives, built-in value objects and those from other execution contexts.

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>).

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.

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:

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 with npm install kindof@1.

Note: Kindof.js follows semantic versioning.

Take the kindof.js file and source it at will.

Install with npm install kindof.
And require with var kindof = require("kindof").

Pass any object to kindof and compare its output to what you expect:

kindof("Hello") // => "string" 
kindof(new Date(2000, 5, 18)) // => "date" 

A switch statement might help:

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.")
}

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.

ValueKindof
undefinedundefined
nullnull
trueboolean
falseboolean
42number
NaNnumber
Infinitynumber
"Hello"string
Symbol()symbol
Symbol("forEach")symbol
Symbol.iteratorsymbol
/.*/regexp
new RegExp(".*")regexp
new Datedate
[42, 69]array
function() {}function
{}object
argumentsobject
new Boolean(true)object
new Number(42)object
new String("Hello")object
new MyClassobject
new Errorobject
Mathobject
JSONobject

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.

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.

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.