Scope Chain in JavaScript

1. What is Scope?

Scope means the area of a program where a variable can be accessed.

let name = "Abishek";

console.log(name);

Here, name can be accessed because it is available in that scope.

2. What is Scope Chain?

Scope Chain is the process JavaScript uses to find a variable.

When JavaScript needs a variable, it first searches in the current scope. If it is not found, it searches in the outer scope. It continues searching until it reaches the global scope.

Simple Flow

Current Scope
      ↓
Outer Scope
      ↓
Global Scope

3. Example of Scope Chain

let a = 10;

function outer() {
    let b = 20;

    function inner() {
        let c = 30;

        console.log(a);
        console.log(b);
        console.log(c);
    }

    inner();
}

outer();

There are three scopes:

Global Scope
     ↓
outer() Scope
     ↓
inner() Scope

Finding c

JavaScript searches:

inner scope → c found ✅

So:

30

Finding b

JavaScript searches:

inner scope → b not found ❌
       ↓
outer scope → b found ✅

So:

20

Finding a

JavaScript searches:

inner scope → a not found ❌
       ↓
outer scope → a not found ❌
       ↓
global scope → a found ✅

So:

10

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4. What if the Variable is Not Found?

If JavaScript searches the entire scope chain and cannot find the variable, it gives a ReferenceError.

function test() {
    console.log(x);
}

test();

JavaScript searches:

test scope  → x ❌
     ↓
global scope → x ❌

Since x is not found:

ReferenceError: x is not defined

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5. Inner Scope Can Access Outer Scope

An inner function can access variables from its outer scope.

function outer() {
    let a = 10;

    function inner() {
        console.log(a);
    }

    inner();
}

outer();

Output:

10

Why?

Because JavaScript searches outward:

inner → outer → global

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6. Outer Scope Cannot Access Inner Scope

An outer function cannot access variables declared inside an inner function.

function outer() {

    function inner() {
        let b = 20;
    }

    console.log(b); // Error
}

b belongs only to the inner scope.

The scope chain does not search inward.

outer → inner ❌

It only searches outward:

inner → outer → global

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7. Important Rule

Scope Chain always searches:

Inside → Outside

or

Current Scope → Parent Scope → Global Scope

It does not search:

Outside → Inside

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The inner scope can access outer variables, but the outer scope cannot access inner variables.

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