JavaScript Scope Explained

In JavaScript, scope defines where variables are accessible. Understanding scope is crucial for writing clean, bug-free, and maintainable code.

Types of JavaScript Scope

1. Global Scope

A variable declared outside any function or block is in the global scope. It can be accessed anywhere in the code.

Example:

let globalVar = "I am global";

function showGlobal() {
  console.log(globalVar);
}

showGlobal();

Output

Avoid polluting global scope—too many global variables can lead to bugs.

2. Local (Function) Scope

Variables declared inside a function using var, let, or const are local to that function.

Example:

function greet() {
  let localVar = "I am local";
  console.log(localVar);
}

greet();
// console.log(localVar);  Error: localVar is not defined

Output

3. Block Scope (let and const only)

Variables declared inside a block ({}), such as inside if, for, or while, are only accessible within that block.

Example:

if (true) {
  let blockVar = "I am block-scoped";
  console.log(blockVar); //  Accessible here
}

// console.log(blockVar);  Error: blockVar is not defined

Output

var is NOT block-scoped – it's function-scoped, which can lead to bugs.

example:

if (true) {
  var testVar = "Not block scoped";
}
console.log(testVar); //  Still accessible outside the block

Output

Scope Table Summary