Explain var let and const in JavaScript with Example.

In JavaScript, var, let, and const are used to declare variables. They differ in scope, reassignability, and hoisting behavior. Let’s break it down:

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1. var

• Introduced: Before ES6 (ES5 and earlier)
• Scope: Function-scoped (available throughout the entire function)
• Hoisting: Hoisted to the top with undefined value
• Reassignable: Yes
• Redeclarable: Yes

Example:

javascriptCopyEditvar x = 10;
function example() {
    var y = 20;
    console.log(y); // 20
}
console.log(x); // 10
// console.log(y); // Error: y is not defined (function scoped)

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2. let

• Introduced: In ES6 (2015)
• Scope: Block-scoped (available only within {})
• Hoisting: Hoisted but not initialized (temporal dead zone)
• Reassignable: Yes
• Redeclarable: No (within the same scope)

Example:

javascriptCopyEditlet a = 30;
if (true) {
    let b = 40;
    console.log(b); // 40
}
console.log(a); // 30
// console.log(b); // Error: b is not defined (block scoped)

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3. const

• Introduced: In ES6 (2015)
• Scope: Block-scoped
• Hoisting: Hoisted but not initialized (temporal dead zone)
• Reassignable: No
• Redeclarable: No

Example:

javascriptCopyEditconst PI = 3.14;
if (true) {
    const GRAVITY = 9.8;
    console.log(GRAVITY); // 9.8
}
console.log(PI); // 3.14
// PI = 3.1416; // Error: Assignment to constant variable

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Summary Table:

Keyword	Scope	Hoisting	Reassignable	Redeclarable
var	Function Scope	Yes	Yes	Yes
let	Block Scope	Yes	Yes	No
const	Block Scope	Yes	No	No

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Key Takeaways:

Avoid var unless working with legacy code.

Prefer let for variables that can change.

Use const for constants or values that don’t change.