I would like to know if there is any way of overriding the behaviour of the typeof operator. Specifically, I want to return "string" when the typeof operator is called both on foo = "hi" and bar = new String("hi").

typeof bar returns "object" but I want it to return "string".

I know this can be done by declaring my own function or accessing the constructor name but I want to modify the behaviour of typeof operator.

EDIT - I am looking for some code that I can add say at the beginning of the program which modifies the behaviour of all the typeof operators in the rest of the program.

I would like to know if there is any way of overriding the behaviour of the typeof operator. Specifically, I want to return "string" when the typeof operator is called both on foo = "hi" and bar = new String("hi").

typeof bar returns "object" but I want it to return "string".

I know this can be done by declaring my own function or accessing the constructor name but I want to modify the behaviour of typeof operator.

EDIT - I am looking for some code that I can add say at the beginning of the program which modifies the behaviour of all the typeof operators in the rest of the program.

• javascript
• typeof

• 3 I'm 99% sure you can't override typeof...just create your own function – Ian Commented Jun 27, 2013 at 2:50
• Thought as much..just wanted to be sure..thanks! – everconfusedGuy Commented Jun 27, 2013 at 2:52
• No problem! Not sure if someone can back me up with documentation, but... – Ian Commented Jun 27, 2013 at 2:54
• 1 Why don't you do typeof obj.valueOf()? It'll return a string primitive for both. – Blender Commented Jun 27, 2013 at 2:57
• @ian looks like typeof is an operator so don't think you can change that: ecma-international/ecma-262/5.1/#sec-11.4.3 – HMR Commented Jun 27, 2013 at 2:58

4 Answers 4

That is impossible. The behaviour of native operators cannot be changed.

Related links:

• Why hasn't operator overloading been added to ECMAScript? at quora.
• The ES value proxy proposal. It doesn't let you change the existing functionality of typeof, but would enable you to define your own additional types.

You can't change a Javascript operator, however you can check if it's a string OR a string object with instanceof.

var strObj = new String('im a string')
var str = 'im a string'

alert(strObj instanceof String); //true
alert(typeof strObj == 'string'); //false
alert(str instanceof String); //false
alert(typeof str == 'string'); //true
alert(strObj instanceof String || typeof strObj == 'string'); //true
alert(str instanceof String || typeof str == 'string'); //true

Of course, it is much more simple and shorter to create your own function, but if you want to use native JS, that is the way : alert(str instanceof String || typeof str == 'string');.

typeof is an operator in JavaScript so I'm quite sure you can't. To detect if something is a string you could use something like this:

var s = "hello";
console.log(s.substr&&s.charAt&&s.toUpperCase==="".toUpperCase)//true
s = new String("hello");
console.log(s.substr&&s.charAt&&s.toUpperCase==="".toUpperCase)//true

No, you can't modify the behavior of typeof or any other operator for that matter. However the next best solution is to use Object.prototype.toString as follows:

function typeOf(value) {
    return Object.prototype.toString.call(value).slice(8, -1);
}

Now you can use it as follows (see the demo - http://jsfiddle/CMwdL/):

var foo = "hi";
var bar = new String("hi");

alert(typeOf(foo)); // String
alert(typeOf(bar)); // String

The reason this works is given in the following link: http://bonsaiden.github.io/JavaScript-Garden/#types.typeof