TypeOf Operator

The typeof operator returns a string indicating the type of the operand's value.

The following table summarizes the possible return values of typeof. For more information about types and primitives, see also the JavaScript data structure page.

Type	Result
Undefined	"undefined"
Null	"object" (reason)
Boolean	"boolean"
Number	"number"
BigInt	"bigint"
String	"string"
Symbol	"symbol"
Function (implements [[Call]] in ECMA-262 terms; classes are functions as well)	"function"
Any other object	"object"

This list of values is exhaustive. No spec-compliant engines are reported to produce (or had historically produced) values other than those listed.

Basic usage

// Numbers
typeof 37 === "number";
typeof 3.14 === "number";
typeof 42 === "number";
typeof Math.LN2 === "number";
typeof Infinity === "number";
typeof NaN === "number"; // Despite being "Not-A-Number"
typeof Number("1") === "number"; // Number tries to parse things into numbers
typeof Number("shoe") === "number"; // including values that cannot be type coerced to a number

typeof 42n === "bigint";

// Strings
typeof "" === "string";
typeof "bla" === "string";
typeof `template literal` === "string";
typeof "1" === "string"; // note that a number within a string is still typeof string
typeof typeof 1 === "string"; // typeof always returns a string
typeof String(1) === "string"; // String converts anything into a string, safer than toString

// Booleans
typeof true === "boolean";
typeof false === "boolean";
typeof Boolean(1) === "boolean"; // Boolean() will convert values based on if they're truthy or falsy
typeof !!1 === "boolean"; // two calls of the ! (logical NOT) operator are equivalent to Boolean()

// Symbols
typeof Symbol() === "symbol";
typeof Symbol("foo") === "symbol";
typeof Symbol.iterator === "symbol";

// Undefined
typeof undefined === "undefined";
typeof declaredButUndefinedVariable === "undefined";
typeof undeclaredVariable === "undefined";

// Objects
typeof { a: 1 } === "object";

// use Array.isArray or Object.prototype.toString.call
// to differentiate regular objects from arrays
typeof [1, 2, 4] === "object";

typeof new Date() === "object";
typeof /regex/ === "object";

// The following are confusing, dangerous, and wasteful. Avoid them.
typeof new Boolean(true) === "object";
typeof new Number(1) === "object";
typeof new String("abc") === "object";

// Functions
typeof function () {} === "function";
typeof class C {} === "function";
typeof Math.sin === "function";

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typeof null

// This stands since the beginning of JavaScript
typeof null === "object";

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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". (reference)

A fix was proposed for ECMAScript (via an opt-in), but was rejected. It would have resulted in typeof null === "null".

Using new operator

All constructor functions called with new will return non-primitives ("object" or "function"). Most return objects, with the notable exception being Function, which returns a function.

const str = new String("String");
const num = new Number(100);

typeof str; // "object"
typeof num; // "object"

const func = new Function();

typeof func; // "function"