JavaScript Fundamentals · Functions · Lesson 15 of 48

Scope

Concept

Scope

Variables declared inside a function only exist inside that function, which is called local scope. Variables declared outside are global.

This isolation is a feature: it means two different functions can each safely use a variable named total without interfering with each other.

By the end of this lesson

Explain the core idea behind Scope.
Predict output before running a change.
Test one realistic and one unusual input.
Use the result to make the next decision.

How to study this page

1. Read one concept.
2. Change the example.
3. Run, compare, and explain.
4. Complete the challenge below.

Syntax

function outer() {
  let local = 1; // only visible inside outer()
}

ExampleRunnable

let outside = "I am global";
function show() {
  let inside = "I am local";
  console.log(outside, inside);
}
show();

Try it Yourself »

Block scope with let

if (true) {
  let blockScoped = "only visible here";
  console.log(blockScoped);
}

Note: let and const are scoped to the nearest set of curly braces, not just to functions — this includes if blocks and loops.

Self-check before continuing

Without looking at the example, describe what changes when you modify one input in Scope. Then reopen the editor and prove your explanation with a small test.

You are ready to continue when you can predict, test, and explain the result.

Your turn

Predict and then test what happens if you try to log the inside variable outside the function.

Loading editor…

Console

Output will appear here...