Q71- What is hoisting in JavaScript? How does it work for var, let, and const?

Hoisting is JavaScript’s default behavior of moving declarations to the top of their scope before code execution. This means variables and functions can be referenced before they are declared, but how they behave depends on the type of declaration.

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1️⃣ Hoisting with var

• Variables declared with var are hoisted and initialized with undefined.
• You can reference the variable before its declaration, but its value is undefined until the assignment line is reached.

console.log(a); // undefined
var a = 10;
console.log(a); // 10

Explanation:
The JS engine internally does this:

var a;       // declaration hoisted
console.log(a); // undefined
a = 10;      // assignment remains in place
console.log(a); // 10

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2️⃣ Hoisting with let

• Variables declared with let are hoisted but not initialized.
• Accessing them before declaration causes a ReferenceError due to the temporal dead zone (TDZ).

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 20;

Explanation:
let and const are in a TDZ from the start of the scope until the declaration line.

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3️⃣ Hoisting with const

• Works like let — hoisted but not initialized.
• Must be initialized at the time of declaration.
• Accessing before declaration or not initializing will throw an error.

console.log(c); // ReferenceError
const c = 30;

const d; // SyntaxError: Missing initializer in const declaration

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4️⃣ Function Hoisting

Function Declarations

• Fully hoisted — you can call them before they appear in code.

sayHi(); // "Hi!"
function sayHi() {
  console.log("Hi!");
}

Function Expressions

• Hoisted only as variables:

console.log(sayHello); // undefined
var sayHello = function() {
  console.log("Hello");
};

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✅ Summary Table

Declaration	Hoisted?	Initialized?	Access before declaration
var	Yes	undefined	Yes, returns undefined
let	Yes	No	No, ReferenceError (TDZ)
const	Yes	No	No, ReferenceError (TDZ)
Function Declaration	Yes	Yes	Yes, can call early
Function Expression	Yes (var)	No	No, undefined if var; TDZ if let/const

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In short:

Hoisting moves declarations to the top. var gets initialized with undefined, while let and const are hoisted but stay uninitialized until the line is reached (TDZ). Functions are fully hoisted if declared normally.