JavaScript let vs const vs var: Complete Variable Declaration Guide

Javascript Study Chapter · Official ReferenceAfzaal Suleman Knowledge Base

JavaScript offers three ways to declare variables: var, let, and const. Understanding their differences in scope, hoisting, and mutability is crucial for writing clean, bug-free code.

Scope Differences

var: Function-scoped, can be redeclared
let: Block-scoped, cannot be redeclared
const: Block-scoped, cannot be redeclared or reassigned

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Code Demonstration & Syntax

javascript

// var - function scoped
function testVar() {
  if (true) {
    var x = 10;
  }
  console.log(x); // 10 - accessible outside block
}

// let - block scoped
function testLet() {
  if (true) {
    let y = 20;
  }
  console.log(y); // ReferenceError - not accessible
}

// const - block scoped, immutable
const PI = 3.14159;
PI = 3.14; // TypeError - cannot reassign

Output

10
ReferenceError: y is not defined
TypeError: Assignment to constant variable

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Chapter Rules & Key Notes

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Remember This

Always prefer const by default, use let when you need reassignment
Avoid var in modern JavaScript code
const objects can have their properties modified (only the reference is immutable)

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Important Rules

Use const for variables that should not be reassigned
Use let for variables that need to be reassigned
Never use var in ES6+ code

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Common Mistakes

Thinking const makes objects completely immutable
Using var instead of let in loops
Forgetting that let/const are block-scoped

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