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JavaScript - How to Use the typeof operator in JavaScript

The typeof operator accepts an operand and returns the type of the provided operand as a string.

The following sample code shows the typeof operator in use:

const str = 'Im a string';
console.log(typeof str);
// Expected output: string

In the example above, const str is a String and serves as the operand for the typeof operator.

The expected output of the typeof operator, when applied to a String, is “string”.

JavaScript variables have different data types, and each data type has a different set of properties and methods.

Furthermore, these data types may interact with one another. When they do so, the results of the interactions are type-dependent. For example, when adding a String to a Number the result will be a concatenation of the two values as if they were both strings, as seen below:

const str = 'Im a string';
const num = 3;
console.log(num + str);
// Expected output: 3Im a string

In the example above, const str is a String, and const num is a Number. When adding these two constants together, the result is a String: ‘3Im a string’.

Syntax

The syntax for applying the typeof operator is straightforward- simply add the typeof keyword to the left of the expression whose type you wish to check. You can also wrap the expression in parentheses, should you so desire.

The syntax of the typeof operator is:

typeof operand

  • operand: an expression whose type you wish to reveal

Examples

The following examples demonstrate the return value of the typeof operator when it is applied to different JavaScript data types

Number

const num = 3;
console.log(typeof num);
// Expected output: number

 

BigInt

const bigNum = 1087874764675896857925344n;
console.log(typeof bigNum);
// Expected output: bigint

 

Boolean

const b = true;
console.log(typeof b);
// Expected output: boolean

 

undefined

const x = undefined;
console.log(typeof x);
// Expected output: undefined

 

Symbol

const sym = Symbol('foo');
console.log(typeof sym);
// Expected output: symbol

 

Object

const obj = {};
console.log(typeof obj);
// Expected output: object

 

Array

const arr = [];
console.log(typeof arr);
// Expected output: object

Note: the type of Array is Object!

Tip: To detect if an expression is an array, use the Array.isArray() method.

console.log(Array.isArray(arr));
// Expected output: true

 

Function

const f = function () { return 'Im a function' };
console.log(typeof f);
// Expected output: function

 

null

const n = null;
console.log(typeof n);
// Expected output: object

Note: the type of null is Object!

 

Related Articles

JavaScript – Data types explained

JavaScript – Objects explained

JavaScript – Array to String

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