TDZ Interview Questions for let and const in JavaScript

TDZ Interview Questions for let and const in JavaScript

The Temporal Dead Zone is a favorite interview topic. Here are the common questions, with answers and code.

Q1: What is the output?

console.log(x);
let x = 5;
``
**Answer**: `ReferenceError: Cannot access 'x' before initialization`. `x` is in the TDZ.

### Q2: What is the output?

```js
console.log(y);
var y = 5;
``
**Answer**: `undefined`. `var` is hoisted with `undefined`.

### Q3: What is the output?

```js
let x = 1;
{
  console.log(x);
  let x = 2;
}
``
**Answer**: `ReferenceError`. The inner `let x` is hoisted (shadowing the outer), so `x` in the block is in the TDZ until the inner declaration line. It does not fall back to the outer `x`.

### Q4: What is the output?

```js
console.log(typeof a);
let a;
``
**Answer**: `ReferenceError`. Even `typeof` cannot escape the TDZ.

### Q5: What is the output?

```js
function foo(a = b, b = 2) {
  console.log(a, b);
}
foo();
``
**Answer**: `ReferenceError: Cannot access 'b' before initialization`. `b` is in the TDZ when `a`'s default is evaluated (defaults run left to right).

### Q6: What is the output?

```js
for (let i = i; i < 3; i++) {}
``
**Answer**: `ReferenceError`. The right-hand `i` is in the TDZ until the declaration completes.

### Q7: What is the output?

```js
const x;
``
**Answer**: `SyntaxError: Missing initializer in const declaration`. This is a parse-time error, not a TDZ error.

### Q8: What is the output?

```js
const x = 5;
x = 10;
``
**Answer**: `TypeError: Assignment to constant variable`. The variable exists (no `ReferenceError`), but reassignment is not allowed.

### Tips for Interviews

- TDZ applies to `let`, `const`, `class`, and default parameters.
- `typeof` does NOT escape the TDZ (unlike undeclared variables).
- `var` is never in the TDZ; it is initialized to `undefined`.
- Shadowing + TDZ: an inner `let` shadows the outer from the start of the block.
- `const` without an initializer is `SyntaxError`, not TDZ.

### The Takeaway

TDZ interview questions test: early access to `let`/`const`, `typeof` in the TDZ, shadowing + TDZ, default parameter TDZ, loop TDZ, `const` without initializer (SyntaxError), and `const` reassignment (TypeError). Walk through the memory phase and the TDZ for each `let`/`const`.