The Temporal Dead Zone (TDZ) is the time between:

  1. When a let or const variable is hoisted, and
  2. When it is actually declared/initialized in code.

During this time, the variable exists in memory (because of hoisting), but you can’t access it — doing so throws a ReferenceError.

📦 Example: 

console.log(x); // ❌ ReferenceError: Cannot access 'x' before initialization
let x = 10;

Even though x is hoisted, it lives in the TDZ from the start of the block until the line let x = 10.

✅ Good Example (Outside TDZ): 

let x = 10;
console.log(x); // ✅ 10

Here, you're accessing it after the TDZ is over — totally valid.

⛔ With var (No TDZ): 

console.log(y); // ✅ undefined
var y = 20;

var is hoisted and initialized with undefined, so there's no TDZ. But this can cause bugs, which is why let and const were introduced.

🧪 With const 

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

const also has a TDZ — and must be initialized at the time of declaration.

🔁 TDZ in a block scope: 

{
  // TDZ starts
  // console.log(a); // ❌ ReferenceError
  let a = 5; // TDZ ends
}

🤯 Why Does TDZ Exist?

The TDZ helps prevent bugs by:

  • Making variables accessible only after explicit declaration
  • Avoiding accidental usage of undefined values
  • Encouraging cleaner, more predictable code