JS-008
Visual JavaScript guide to undefined: Learn what undefined means in JavaScript
Beginner2 min~2 min read#undefined#primitive-types#variables

undefined

Not assigned yet

Goal

Learn what undefined means in JavaScript.

Explanation

  • undefined means a value has not been assigned yet.
  • If you create a variable without a value, JavaScript automatically assigns undefined to it.
  • Unlike null, undefined does not need to be written manually.
  • This means the variable exists but does not contain anything yet.
  • undefined is common while writing and debugging programs.

Analogy

Imagine an empty box that nothing has been put into yet. Nobody has even decided what should go inside. That is undefined.

Code

let age;

console.log(age);

Result: undefined

Common mistake

Many beginners think undefined and null mean the same thing. In reality, they serve different purposes.

Remember

  • ✦ undefined means a value has not been set yet.
  • ✦ JavaScript can assign undefined automatically.
  • ✦ undefined and null are different values.

Quick Quiz

When does JavaScript automatically return undefined?

Resources