Scope
Scope decides where variables are visible.
Syntax
function f() { let local = 1; }Scope determines where a variable can be used.
- Global scope — declared outside any function; visible everywhere.
- Function/block scope — declared inside
{ }; visible only there.
Code inside a function can read outer variables, but the outside cannot see variables declared inside the function.
Example
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When to use it
- Keep a helper variable private inside a function so it cannot interfere with global module state.
- Understand why a loop variable declared with var is shared across all iteration closures.
- Use a nested function to encapsulate intermediate logic that only makes sense within the outer function.
More examples
Local variable is function-private
Declares tax inside a function, making it inaccessible outside and preventing naming collisions.
function processOrder() {
const tax = 0.08; // local to processOrder
return 100 * (1 + tax);
}
console.log(processOrder()); // 108
// console.log(tax); // ReferenceErrorNested function scope
Shows that an inner function can read variables from its enclosing outer function via the scope chain.
function outer() {
const x = 10;
function inner() {
const y = 20;
return x + y; // inner can read outer's x
}
return inner();
}
console.log(outer()); // 30Block scope vs function scope
Contrasts var's function scope (one a across the function) with let's block scope (two separate b variables).
function demo() {
var a = "function-scoped";
let b = "block-scoped";
if (true) {
var a = "still the same a";
let b = "different b";
}
console.log(a); // "still the same a"
console.log(b); // "block-scoped"
}
demo();
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