❮ JS Object Constructors JS Code Blocks ❯

JavaScript Scope

Scope determines where variables are accessible. JavaScript has three types of scope: Global, Function, and Block.

Global Scope

Variables declared outside any function or block are in the global scope. They are accessible from anywhere in the script.

Global Scope

let globalVar = "I am global";

function showGlobal() {
    console.log(globalVar);   // accessible inside function
}

showGlobal();
console.log(globalVar);       // accessible outside too

Function Scope

Variables declared inside a function are local to that function. They cannot be accessed from outside.

Function Scope

function greet() {
    let message = "Hello!";   // local to greet()
    console.log(message);
}

greet();
// console.log(message);  // ReferenceError — not accessible here

Block Scope (let and const)

Variables declared with let or const inside a block ({}) are only accessible within that block.

Block Scope

if (true) {
    let blockVar   = "block scoped";
    const blockConst = "also block scoped";
    console.log(blockVar);      // OK inside block
}
// console.log(blockVar);   // ReferenceError outside block

The Scope Chain

Inner scopes can access outer scope variables, but outer scopes cannot access inner scope variables.

Scope Chain

let outer = "outer";

function middle() {
    let mid = "middle";

    function inner() {
        let innerVar = "inner";
        console.log(outer);     // ✓ can access outer
        console.log(mid);       // ✓ can access middle
        console.log(innerVar);  // ✓ can access own
    }
    inner();
    // console.log(innerVar);  // ✗ ReferenceError
}
middle();

Variable Shadowing

When an inner scope declares a variable with the same name as an outer scope variable, it shadows the outer one.

Shadowing

let color = "red";

function paint() {
    let color = "blue";   // shadows outer 'color'
    console.log(color);    // "blue"
}

paint();
console.log(color);        // "red" — outer unchanged

Scope	Declared With	Accessible From
Global	Outside all blocks	Anywhere in the script
Function	Inside a function	Only within that function
Block	let/const in a block	Only within that block

📝 Note: var does NOT respect block scope — it leaks out of if/for blocks. Always use let or const for predictable scoping.

Exercise:

Variables declared outside any function or block have which scope?

Block scopeFunction scopeGlobal scopeLocal scope

❮ JS Object Constructors JS Code Blocks ❯