A proper and semantic typeof function that works with literal primitives, boxed objects and those from another execution context.


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>).

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: , 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.

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 String("Hello")) // => "string" 

A switch statement might help:

switch (kindof(obj)) {
  case "null": = "Alfred"; break
  case "string": = obj; break
  case "number": this.age = obj; break
  default: throw new TypeError("Pardon, sir, came upon an unexpected type.")

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.

new Boolean(true)boolean
new Number(42)number
new String("Hello")string
new RegExp(".*")regexp
new Datedate
[42, 69]array
function() {}function
new MyClassobject
new Errorobject

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 or create an issue online.