JavaScript typeof Operator – How to Check Data Types

In JavaScript, the typeof operator is used to determine the type of a given variable or value. It helps developers identify what kind of data they are working with—such as a string, number, boolean, object, or function—making it a key part of debugging and writing reliable code.

Syntax

typeof operand
// or
typeof(operand)

Example:

typeof "Hello";  // "string"
typeof 123;      // "number"
typeof true;     // "boolean"

Common Return Values of typeof

Type	Example	typeof Result	Description
String	"JavaScript"	"string"	Sequence of characters.
Number	42, 3.14	"number"	Integer or floating-point number.
BigInt	123n	"bigint"	For arbitrarily large integers.
Boolean	true, false	"boolean"	Logical true/false value.
Undefined	undefined	"undefined"	Variable declared but not assigned a value.
Object	{}, [1,2,3], null	"object"	Collections, arrays, or null (special case).
Function	function() {}	"function"	Any callable object or function.
Symbol	Symbol('id')	"symbol"	Unique and immutable primitive value.

Examples of typeof in Action

Primitive Data Types

console.log(typeof "John");        // "string"
console.log(typeof 25);            // "number"
console.log(typeof true);          // "boolean"
console.log(typeof undefined);     // "undefined"
console.log(typeof 123n);          // "bigint"
console.log(typeof Symbol("id"));  // "symbol"

Non-Primitive (Reference) Data Types

console.log(typeof { name: "Alice" }); // "object"
console.log(typeof [1, 2, 3]);         // "object" (arrays are also objects)
console.log(typeof null);              // "object" (this is a long-standing JavaScript bug)
console.log(typeof function(){});      // "function"

Special Cases and Gotchas

typeof null → "object"

This is a well-known JavaScript quirk from its early implementation.

Even though null is a primitive type, typeof null incorrectly returns "object".

console.log(typeof null); // "object"

Arrays

typeof returns "object" for arrays too, even though they are special objects:

console.log(typeof [1,2,3]); // "object"

To specifically check for an array, use:

Array.isArray([1,2,3]); // true

NaN (Not a Number)

console.log(typeof NaN); // "number"

typeof with undeclared variables

Unlike most operations, using typeof on an undeclared variable does not throw an error — it safely returns "undefined":

console.log(typeof undeclaredVar); // "undefined"