1. var — “The old-school way”

Scope:

• Function-scoped
  • var is function-scoped, meaning the variable is accessible throughout the entire function in which it is declared (even before the declaration, due to hoisting). If declared outside of any function, it’s globally scoped.
  • Not limited by { } blocks like if, for, or while.

function test() {
  if (true) {
    var x = 10;
  }
  console.log(x); // ✅ 10 — still accessible!
}

Redeclaration:

• Allowed in the same scope.

var a = 1;
var a = 2; // ✅ No error

Reassignment:

• Allowed.

var a = 1;
a = 2; // ✅ Works fine

🔷 Hoisting:

• Yes, variables are hoisted to the top of their function.
• Initialized as undefined during hoisting.
• Script not stopped and it execute continue.

function test() {
  console.log(a); // ✅ undefined
  var a = 5;
}

2. let — “Modern and safer”

🔷 Scope:

• Block-scoped
  • Only accessible inside the { } block it’s declared in.

if (true) {
  let x = 10;
}
console.log(x); // ❌ ReferenceError

Output:

10
Uncaught ReferenceError: a is not defined

Hoisting:

• Yes, hoisted to the top of the block.
• ❗But not initialized — accessing it before declaration causes a ReferenceError.
• And below code is not executable.

console.log(a); // ❌ ReferenceError
let a = 5;
console.log("I am not executed.");

Output:

Uncaught ReferenceError: a is not defined

Redeclaration:

• ❌ Not allowed in the same scope.
• SyntaxError will show at line 2 and below code is not executable.

let a = 1;
let a = 2; // ❌ SyntaxError
console.log(a); // I am not executable due to above SyntaxError

Output:

Uncaught ReferenceError: a is not defined

Reassignment:

• ✅ Allowed.

let a = 1;
a = 2; // ✅ Works fine
console.log(a);

Output:

2

3. const — “Constant value”

🔷 Scope:

• Block-scoped
  • Just like let.

if (true) {
  const x = 10;
}
console.log(x); // ❌ ReferenceError

Output:

10
Uncaught ReferenceError: a is not defined

Hoisting:

• Yes, but not initialized.
• ❗Accessing before declaration causes a ReferenceError.
• And below code is not executable.

console.log(a); // ❌ ReferenceError
const a = 5;
console.log("I am not executable.");

Output:

Uncaught ReferenceError: a is not defined

Redeclaration:

• ❌ Not allowed in the same scope.
• SyntaxError will show at line 2 and below code is not executable.

const a = 1;
const a = 2; // ❌ SyntaxError
console.log(a); // I am not executable due to above SyntaxError

Output:

Uncaught ReferenceError: a is not defined

Reassignment:

• ❌ Not allowed.
• Must be initialized immediately, and value cannot be changed.

const a = 1;
a = 2; // ❌ TypeError
console.log(a); // I am not executable due to above TypeError
console.log("I am also not executable line due to above TypeError");

Output:

Uncaught TypeError: Assignment to constant variable.

✅ Exception:

• If const holds an object or array, the contents can be changed.

const obj = { a: 1 };
obj.a = 2; // ✅ Allowed — only the reference is constant

Quick Comparison Table

✅ Mini Quiz

Q1: What will be the output?

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

Q2: What will this do?

console.log(b);
let b = 10;

Q3: Can this work?

const c = 5;
c = 6;

Q4: What happens here?

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

Q5: Will this throw an error?

let x = 1;
let x = 2;