What is the Difference Between null and undefined in JavaScript?

In JavaScript, both null and undefined represent “no value,” but they are used in different scenarios and have subtle differences that are important to understand.

undefined – A Variable That Has Been Declared But Not Assigned

undefined is the default value assigned to:

• Variables that are declared but not initialized
• Function parameters that are not passed
• Missing properties in an object

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

function greet(name) {
  console.log(name);
}
greet(); // undefined

const person = {};
console.log(person.age); // undefined

null – A Value That You Intentionally Set

null is used when you want to intentionally assign “no value” to a variable. It represents the intentional absence of any object value.

let selectedColor = null; // No color selected yet

Type Comparison

Both null and undefined are falsy values in JavaScript:

Boolean(null);      // false
Boolean(undefined); // false

But their types are different:

typeof null;        // "object" (this is a historical bug in JS)
typeof undefined;   // "undefined"

Equality Comparison

null == undefined;  // true (loose equality)
null === undefined; // false (strict equality)

When to Use What?

• Use undefined when a variable hasn’t been initialized.
• Use null when you want to explicitly indicate “no value.”