What is the difference between typeof and instanceof?

The difference between typeof and instanceof in JavaScript is subtle but very important. Let’s break it down clearly:

1. typeof

• Purpose: Checks the data type of a variable or value.
• Returns: A string indicating the type.
• Syntax: typeof value
• Common returned values: "undefined", "boolean", "number", "string", "bigint", "symbol", "function", "object"
  • Example:

typeof 42; // "number" 
typeof "hello"; // "string" 
typeof true; // "boolean" 
typeof {}; // "object" 
typeof null; // "object" (quirky behavior) 
typeof undefined; // "undefined" 
typeof function(){}; // "function"

• Notes:
  • Works on primitives.
  • Cannot tell object types apart (e.g., array vs object).

2. instanceof

• Purpose: Checks if an object is an instance of a constructor or class.
• Returns: A boolean (true or false).
• Syntax: object instanceof Constructor
• Example:

const arr = [1, 2, 3]; 
arr instanceof Array; // true 
arr instanceof Object; // true 
arr instanceof String; // false 
const date = new Date(); 
date instanceof Date; // true 
date instanceof Object; // true

• Notes:
  • Works only on objects, not primitives.
  • Checks prototype chain.
  • Useful to distinguish between object types like Array, Date, Map, etc.

Quick Comparison Table

✅ Rule of thumb:

• Use typeof for primitives (number, string, boolean, etc.)
• Use instanceof for objects to check their class or constructor.