Scope in JavaScript

Scope determines where variables are accessible in your code. It defines the 'visibility' and lifetime of variables – whether you can use a variable in a certain part of the program.

JavaScript has three main types of scope: global, function, and block. The rules changed with ES6 (let and const introduce block scope).

Global Scope

Variables declared outside any function or block are global – accessible everywhere in the code.

Global variables should be used carefully – they can cause conflicts in larger programs.

Function Scope

Variables declared with var inside a function are function-scoped – visible only inside that function.

Block Scope (let and const)

Variables declared with let or const inside {} braces (blocks like if, for, while) are block-scoped – only accessible inside that block.

var does NOT have block scope – it leaks out.

Scope Chain

When looking for a variable, JavaScript checks the current scope first, then the parent scope, up to global.

Quick Quiz