Explained Global, Local & Block Scope in JavaScript

Source: Dev.to

Scope

Scope generally means the visibility and accessibility of variables in different parts of your code. Understanding the different types of scopes helps write error‑free and clear code.

Global Scope

A variable declared in the global scope is accessible anywhere in the program, including inside functions and blocks.

let global = "I'm a global variable";

function display() {
  console.log(global);
}

display(); // "I'm a global variable"

Local Scope

Variables declared inside a function are only accessible within that function.

function local() {
  var local = "I'm a local scope variable";

  if (true) {
    console.log(local); // Works here also inside the if block
  }
}

Block Scope

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

if (true) {
  let block = "I'm a block variable";
  console.log(block); // Works here inside the block only
}

// console.log(block); // Uncaught ReferenceError: block is not defined

How to declare variables in JavaScript

Variables can be declared using var, let, or const.

• var – function‑scoped; can be redeclared and reassigned.
• let – block‑scoped; can be reassigned but not redeclared.
• const – block‑scoped; cannot be redeclared or reassigned.

var

console.log("Nikita Maharana");

var a = 25;
var a = 3;   // redeclaration allowed
a = 56;      // reassignment allowed

console.log(a); // 56

let

let x = 9;

// let x = 5; // ❌ redeclaration not allowed
x = 78;      // reassignment allowed

console.log(x); // 78

const

const y = 11;

// const y = 22; // ❌ redeclaration not allowed
// y = 30;       // ❌ reassignment not allowed

console.log(y); // 11

Examples

Example 1 – var is function scoped

function fun() {
  if (true) {
    var a = 55;
  }
  console.log(a); // 55 – accessible throughout the function
}

fun();

Example 2 – let and const are block scoped

function fun() {
  if (true) {
    var a = 55;
    let b = 34;
    const c = 34;
  }

  console.log(a, b); // a is 55, b is ReferenceError
  console.log(c);    // ReferenceError – c is block scoped
}

fun();