JavaScript types

Question

As per http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf, JavaScript has 6 types: undefined, null, boolean, string, number and object.

var und; console.log(typeof und); // <-- undefined  var n = null; console.log(typeof n); // <--- **object**!  var b = true; console.log(typeof b); // <-- boolean  var str = "myString" console.log(typeof str); // <-- string  var int = 10; console.log(typeof int); // <-- number  var obj = {} console.log(typeof obj); // <-- object 

Question 1:

Why is null of type object instead of null?

Question 2:

What about functions?

var f = function() {}; console.log(typeof f); // <-- function 

Variable f has type of function. Why isn't it specified in the specification as a separate type?

Thanks,

Answer 1

About typeof null == 'object', this is a mistake that comes since the early days, and unfortunately this mistake will stay with us for a long time, it was too late to be fixed in the ECMAScript 5th Edition Specification.

About the functions, they are just objects, but they have an special internal property named [[Call]] which is used internally when a function is invoked.

The typeof operator distinguish between plain objects and functions just by checking if the object has this internal property.