Typeof JavaScript

A typeof keyword returns a string indicating the Data Type of the identifier. We use it for getting the type of primitive values.

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.

typeof identifier
typeof (identifier)

For Example

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.

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.

Typeof null is object

Although the null is a data type in JavaScript, typeof null returns “object”

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’.

References

1. Reference
2. The history of “typeof null”
3. Typeof Null Proposal