Advanced

Scope

Where variables are visible: global, function, block.

1. Explanation

Scope determines where a variable can be accessed. Global scope = anywhere. Function scope = inside the function. Block scope = inside { } (only for let/const). Inner scopes can read outer variables (closure), but outer scopes cannot read inner ones.

2. Example

Try it yourself

let global = "I'm everywhere";

function outer() {
  let inFunc = "inside outer";
  if (true) {
    let inBlock = "inside block";
    console.log(global, inFunc, inBlock); // all three work
  }
  // console.log(inBlock); // ❌ ReferenceError
}
outer();

Console

Click Run to see output.

3. Expected output

output

I'm everywhere inside outer inside block

4. Common mistakes

✗Polluting the global scope by forgetting let/const
✗Expecting var to be block-scoped — it isn't
✗Shadowing a variable accidentally with the same name in an inner scope

5. Practice questions

Demonstrate block scope by declaring 'let x' inside an if-block
Show how var leaks out of a for-loop but let doesn't
Write a function that captures an outer variable

→ Try more practice problems