A typeof keyword returns a string indicating the Data Type of the identifier. We use it for getting the type of primitive values.
Table of Contents
Using Typeof
The syntax of the Typeof is as shown below. It returns the data type of the identifier, which we place right next to it.
1 2 3 4 | typeof identifier typeof (identifier) |
For Example
1 2 3 4 | var str="hello world" console.log(typeof str) //string |
Typeof correctly returns the type in case of number, string, boolean, symbol, undefined, function. Everything else is object. That also includes null.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | console.log(typeof 1337) // number console.log(typeof NaN) // number console.log(typeof Infinity) // number console.log(typeof Number('1')) // number console.log(typeof 512n)) // bigint console.log(typeof "foo") // string console.log(typeof true) // boolean console.log(typeof {}) // object console.log(typeof Math.round) // function console.log(typeof Symbol()) // symbol console.log(typeof undefined) // undefined console.log(typeof null) // object console.log(typeof [1, 2, 3]) // object console.log(typeof Math) // object console.log(typeof new Boolean(true)) // object console.log(typeof new Number(10)) // object console.log(typeof new String('xyz')) // object |
The following table summarizes the possible return values of typeof.
| Type | Result |
|---|---|
| Undefined | “undefined” |
| Null | “object” |
| Boolean | “boolean” |
| Number | “number” |
| BigInt | “bigint” |
| String | “string” |
| Symbol | “symbol” |
| Function object | “function” |
| Everything Else | “object” |
Typeof null is object
Although the null is a data type in JavaScript, typeof null returns “object”
1 2 3 | console.log(typeof null) // object |
In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the typeof return value “object”
A fix was proposed for ECMAScript (via an opt-in), but was rejected. It would have resulted in typeof null === ‘null’.
