Difference Between Null and Undefined in JavaScript

In JavaScript, both null and undefined represent the absence of a value, but they serve distinct purposes in code execution. While undefined is the default runtime value indicating that a variable has been declared but not yet assigned, null is an explicit assignment used by developers to represent intentional emptiness. This guide covers their definitions, type representations, behavior in equality operations, and practical differences.

1. What is undefined?

undefined means a variable has been declared, but no value has been assigned to it. It is JavaScript’s default value for uninitialized states.

You encounter undefined when: * A variable is declared using let or var without an initial value. * A function does not explicitly return a value. * You access an object property or array index that does not exist. * Function parameters are omitted during a function call.

let a;
console.log(a); // undefined

function test() {}
console.log(test()); // undefined

const obj = {};
console.log(obj.fakeProperty); // undefined

2. What is null?

null is an intentional assignment. It represents the deliberate absence of any object value or an empty state. JavaScript never sets a value to null automatically; it must be assigned programmatically.

let user = null; // Explicitly set to represent "no user"

3. Type Differences (typeof)

The typeof operator reveals a fundamental difference between the two primitives:

• typeof undefined returns "undefined".
• typeof null returns "object".

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

Note: typeof null === "object" is a legacy bug in JavaScript that cannot be fixed without breaking existing web applications.

4. Equality Comparisons (== vs ===)

When comparing null and undefined, the result depends on whether you use loose or strict equality:

• Loose Equality (==): Returns true because both are falsy values representing emptiness.
• Strict Equality (===): Returns false because they are different data types.

console.log(null == undefined);  // true
console.log(null === undefined); // false

5. Arithmetic Operations

When converted to numbers, null and undefined behave differently:

• null converts to 0.
• undefined converts to NaN (Not a Number).

console.log(null + 5);      // 5   (0 + 5)
console.log(undefined + 5); // NaN (NaN + 5)

Summary of Differences