1. var vs let vs const

var

• Function-scoped.

• Can be redeclared and updated.

• Hoisted to the top of the scope → initialized with undefined.

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

---

let

• Block-scoped.

• Can be updated, but not redeclared in the same scope.

• Hoisted but not initialized → stays in the Temporal Dead Zone (TDZ) until the declaration line.

console.log(b); // ❌ ReferenceError (TDZ)
let b = 20;

---

const

• Block-scoped.

• Must be declared + initialized at the same time.

• Cannot be updated or redeclared.

• Also lives in the TDZ before initialization.

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

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2. 🔥 What is the Temporal Dead Zone (TDZ)?

• The time between hoisting (when memory is allocated) and initialization (actual value assigned).

• For let and const, variables exist in memory but are in a “dead zone” → accessing them gives ReferenceError.

👉 Why? To prevent using variables before they are safely initialized.

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3. Deep Example

// Global Scope
console.log(a); // undefined
// console.log(b); // ReferenceError (TDZ)
// console.log(c); // ReferenceError (TDZ)

var a = 1;
let b = 2;
const c = 3;

function test() {
    // TDZ for x, y, z starts here
    // console.log(x, y, z); // ReferenceError for y, z

    var x = "var inside";
    let y = "let inside";
    const z = "const inside";

    console.log(x); // "var inside"
    console.log(y); // "let inside"
    console.log(z); // "const inside"
}
test();

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4. Scope & Block Example

{
    var v = "I am var";
    let l = "I am let";
    const c = "I am const";
}

console.log(v); // ✅ "I am var"
// console.log(l); // ❌ ReferenceError
// console.log(c); // ❌ ReferenceError

👉 var escapes out of the block → only function-scoped.
👉 let and const stay inside the block → block-scoped.

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5. 🧠 Why TDZ Exists?

• To make let and const safer than var.

• If TDZ didn’t exist:

    console.log(x); // would be undefined
    let x = 10;
  

  This could cause bugs because you might accidentally use an uninitialized variable.

• TDZ forces you to declare before use.

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6. Tree Diagram (Execution Context Example)

// Global LE
{
   a: undefined  (var)
   b: uninitialized (TDZ)
   c: uninitialized (TDZ)
}

Before execution:

• a is hoisted → undefined

• b and c are hoisted too but stay uninitialized (TDZ)

When JS reaches:

• var a = 1; → initializes

• let b = 2; → initializes

• const c = 3; → initializes