Understanding var, let, and const in JavaScript

JavaScript provides three ways to declare variables: var, let, and const.
Though they may look similar, they behave very differently. Understanding these differences helps you write cleaner and more predictable code.

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1. var

var was the original way to declare variables in JavaScript.

Characteristics

• Function-scoped
• Can be redeclared
• Can be reassigned
• Hoisted and initialized as undefined

Example

var name = "Alice";
var name = "Bob";
name = "Charlie";

console.log(name);

Scope Issue

if (true) {
  var x = 10;
}

console.log(x); // 10

2. let

let was introduced in ES6 and is block-scoped.

Characteristics

• Block-scoped
• Cannot be redeclared in the same scope
• Can be reassigned
• Hoisted but not initialized (Temporal Dead Zone)

Example

let age = 25;
age = 26;

// let age = 30; // Error

Block Scope

if (true) {
  let y = 20;
}

// console.log(y); // ReferenceError

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3. const

const is used for values that should not be reassigned.

Characteristics

• Block-scoped
• Cannot be redeclared
• Cannot be reassigned
• Must be initialized when declared

Example

const PI = 3.14;
// PI = 3.14159; // Error

Objects and Arrays

const user = { name: "John" };
user.name = "Jane";

const numbers = [1, 2, 3];
numbers.push(4);

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4. Comparison Table

Feature	var	let	const
Scope	Function	Block	Block
Redeclare	Yes	No	No
Reassign	Yes	Yes	No
Hoisting	Yes	TDZ	TDZ

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5. Best Practices

• Use const by default
• Use let when reassignment is needed
• Avoid var in modern JavaScript

Example

const appName = "My App";
let counter = 0;

counter++;

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Conclusion

• var is outdated and error-prone
• let is safer for mutable values
• const is best for fixed references

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