What is the Temporal Dead Zone in JavaScript?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Block scope starts

   When execution enters a block, let and const names in it are hoisted and reserved, but left uninitialised.

2. Step 2

   Dead zone begins

   From the top of the block until the declaration line, the variable is in the TDZ and any access throws a ReferenceError.

3. Step 3

   Declaration executes

   When the line with let or const runs, the variable is initialised (to its value or undefined for a bare let).

4. Step 4

   Dead zone ends

   After initialisation the variable can be read and written normally for the rest of the block.

5. Step 5

   Contrast with var

   var is initialised to undefined at hoist time, so it has no TDZ and returns undefined instead of throwing.

Code Example

{
  // TDZ for 'price' starts here
  console.log(typeof price); // ReferenceError, not 'undefined'
  console.log(price);        // ReferenceError: Cannot access 'price' before initialization

  let price = 100;           // TDZ ends here; 'price' is now usable
  console.log(price);        // 100
}

// Contrast with var — no TDZ
{
  console.log(qty); // undefined (hoisted and initialised)
  var qty = 5;
  console.log(qty); // 5
}

Follow-up Questions

• Are let and const hoisted? Explain what actually happens.
• Why does typeof throw for a TDZ variable but not for a truly undeclared one?
• How does the TDZ differ from var's behaviour?
• Does function-scope or block-scope define the TDZ boundaries?
• How can the TDZ help you catch bugs during development?