What is the Temporal Dead Zone (TDZ), and how does it show up in interviews?

Short Interview Answer

TDZ is the period from entering a scope until a `let`/`const`/class binding is initialized; accessing it throws ReferenceError.

Detailed Explanation

When a block is entered, `let`/`const` bindings exist in the environment but are uninitialized. Any read or write before the declaration statement runs throws ReferenceError — that window is the TDZ. It also affects default parameters that reference later parameters, and `typeof` on an undeclared `let` (unlike undeclared globals, `typeof` does not safely return `'undefined'` for TDZ bindings). TDZ exists to catch use-before-define bugs that `var`'s silent `undefined` hid.

Example

function f(a = b, b = 2) {
  return a + b;
}
f(); // ReferenceError: Cannot access 'b' before initialization

Interview Tip

Use the default-parameter example — it separates people who memorized TDZ from people who understand it.

Common Mistake

Thinking TDZ means the variable doesn't exist at all; the binding exists but is uninitialized.