Common Hoisting Interview Questions in JavaScript

Common Hoisting Interview Questions in JavaScript

Hoisting is asked in almost every JavaScript interview. Here are the most common questions, with answers and code you can practice.

Q1: What is the output?

console.log(a);
var a = 5;

Answer: undefined. a is hoisted with undefined. The assignment runs at the line.

Q2: What is the output?

foo();
function foo() {
  console.log("hi");
}

Answer: "hi". Function declarations are hoisted with their full body.

Q3: What is the output?

bar();
var bar = function () {
  console.log("hi");
};

Answer: TypeError: bar is not a function. bar is hoisted as undefined.

Q4: What is the output?

console.log(x);
let x = 5;

Answer: ReferenceError: Cannot access 'x' before initialization. let is in the TDZ.

Q5: What is the output?

var a = 1;
function foo() {
  console.log(a);
  var a = 2;
  console.log(a);
}
foo();
console.log(a);

Answer: undefined, 2, 1. Inside foo, the local var a is hoisted (shadowing the outer one), so the first log is undefined. Then a = 2 runs. The outer a is untouched.

Q6: What is the output?

function foo() {
  console.log("1");
}
foo();
function foo() {
  console.log("2");
}
foo();

Answer: "2", "2". The second function declaration overrides the first during hoisting.

Q7: What is the output?

console.log(typeof x);
var x;

Answer: "undefined". typeof on a hoisted var returns "undefined" without throwing.

Q8: What is the output?

console.log(typeof y);
let y;

Answer: ReferenceError. Even typeof cannot escape the TDZ.

Tips for Interviews

• Walk through the memory phase first (what gets hoisted to what value).
• Then walk through the code execution phase line-by-line.
• Mention TDZ whenever let/const is involved.
• Mention that function expressions follow variable hoisting rules, not function declaration rules.

The Takeaway

Hoisting interview questions test the memory phase, the TDZ, shadowing, and the difference between function declarations and expressions. Walk through both phases (memory then execution) for each question.