JavaScript

typeof operator

The typeof operator in JavaScript returns a string indicating the data type of an operand. Let me break down how it works with different types:

1// Numbers (regular and NaN)
2typeof 42; // "number"
3typeof 3.14; // "number"
4typeof NaN; // "number"
5
6// Strings
7typeof 'hello'; // "string"
8typeof ''; // "string"
9
10// Booleans
11typeof true; // "boolean"
12typeof false; // "boolean"
13
14// Undefined
15typeof undefined; // "undefined"
16
17// Objects and null
18typeof {}; // "object"
19typeof null; // "object" (this is a known JavaScript quirk)
20typeof []; // "object" (arrays are objects in JavaScript)
21
22// Functions
23typeof function () {}; // "function"
24
25// Symbol (added in ES6)
26typeof Symbol(); // "symbol"
27
28// BigInt (added in ES2020)
29typeof 42n; // "bigint"

There are a few important gotchas to be aware of:

typeof null returns "object", which is a historical bug in JavaScript
Arrays are considered objects, so typeof [] returns "object"
You can use typeof on undeclared variables without causing an error, it will return "undefined"

The operator is particularly useful for type checking and defensive programming, like checking if a variable exists before using it:

1if (typeof someVariable !== 'undefined') {
2  // safe to use someVariable
3}

What does typeof operator do?

typeof operator

Related JavaScript interview topics