undefined

undefined is the value JavaScript automatically gives to a variable that has been declared but not yet assigned a value. It means "nothing has been put here yet" — and unlike null, you rarely assign it yourself.

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

undefined also shows up automatically in other places — for example, when you access an object property that doesn't exist, or call a function that doesn't explicitly return anything.

const user = { name: 'Ada' };
console.log(user.age); // undefined

The key difference: undefined means a value was never assigned — it's JavaScript's own default for "empty." null means a developer deliberately set "no value" on purpose. Think of undefined as "empty by default" and null as "emptied intentionally."

let a;
let b = null;
console.log(a); // undefined - never assigned
console.log(b); // null - deliberately set to "nothing"

A variable that has been declared but never given a value automatically holds ____.

What is the key difference between null and undefined?

Accessing a property that doesn't exist on an object returns undefined rather than causing an error.