Hoisting, TDZ & Scope: Interview Essentials

Hoisting

JavaScript moves declarations to the top of their scope during compilation (conceptually).

console.log(x); // undefined (not ReferenceError)
var x = 5;

console.log(y); // ReferenceError
let y = 5;

var vs let vs const

Temporal Dead Zone (TDZ)

The period between entering scope and declaration where let/const cannot be accessed.

Interview Questions

Q: What gets hoisted?

• var declarations (initialized as undefined)
• function declarations (fully hoisted)
• let/const (hoisted but in TDZ)

Q: What's the output?

var a = 1;
function test() {
  console.log(a);
  var a = 2;
}
test(); // undefined

Local var a is hoisted inside test, shadowing the outer a.

Best Practice Answer

"Always use const by default, let when reassignment is needed, avoid var in modern code."