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typeof Operator

typeof Operator

Introduction

The typeof operator returns a string indicating the type of the operand. It’s the most common way to check types at runtime.

Why Do We Need This?

Type checking is essential for validation, debugging, and writing type-safe code — especially since JavaScript is dynamically typed.

Visual Explanation

flowchart TD
    Expression["typeof operand"]
    Types["Returns string"]
    S1["'undefined'"]
    S2["'boolean'"]
    S3["'number'"]
    S4["'string'"]
    S5["'symbol'"]
    S6["'bigint'"]
    S7["'function'"]
    S8["'object'"]

    Expression --> Types
    Types --> S1
    Types --> S2
    Types --> S3
    Types --> S4
    Types --> S5
    Types --> S6
    Types --> S7
    Types --> S8

Syntax

typeof operand;
typeof(operand); // function-like syntax

Return Values

typeof undefined;     // 'undefined'
typeof true;          // 'boolean'
typeof 42;            // 'number'
typeof 'hello';       // 'string'
typeof 123n;          // 'bigint'
typeof Symbol();      // 'symbol'
typeof function(){};  // 'function'
typeof {};            // 'object'
typeof [];            // 'object'
typeof null;          // 'object' — infamous bug!

The typeof null Bug

typeof null; // 'object' — this is a bug from JavaScript's first release
// To check for null:
value === null; // use strict equality

Practical Usage

function add(a, b) {
  if (typeof a !== 'number' || typeof b !== 'number') {
    throw new Error('Both arguments must be numbers');
  }
  return a + b;
}

Summary

typeof returns a string indicating the type
typeof null returns 'object' (historical bug)
Arrays return 'object' — use Array.isArray() instead

Further Reading

MDN: typeof