JS Interview Questions
In this page:
Basic Interview Questions
Common opening interview questions probe fundamentals like the difference between var/let/const, how == differs from ===, and what undefined versus null actually mean, since these reveal how solid someone's grasp of the basics really is.
Example: Basic Interview Questions
console.log(typeof undefined, typeof null); // classic var/let/const & null-vs-undefined question
console.log(1 == "1", 1 === "1");Functions and Scope
Function and scope questions often ask you to explain closures, hoisting, or what this refers to in different call contexts — areas where JavaScript's behavior surprises many developers.
Example: Functions and Scope
function outer() {
let count = 0;
return () => ++count; // closures question
}
const counter = outer();
console.log(counter(), counter());Arrays and Objects
Array and object questions typically involve manipulating data with map/filter/reduce, explaining shallow vs. deep copies, or predicting output involving reference vs. value semantics.
Example: Arrays and Objects
const nums = [1, 2, 3];
const doubled = [...nums].map(n => n * 2); // shallow copy question
console.log(nums, doubled);Async JavaScript
Async JavaScript questions usually cover the event loop, the difference between microtasks and macrotasks, and converting callback-based or Promise-chain code to async/await.
Example: Async JavaScript
console.log("1");
setTimeout(() => console.log("2 - macrotask"), 0);
Promise.resolve().then(() => console.log("3 - microtask"));
console.log("4"); // classic event loop ordering questionCoding Interview Practice
Practicing timed coding problems that combine these concepts — not just reading about them — is what actually builds the speed and confidence needed in a live interview setting.
Example: Coding Interview Practice
function getValue(cb) { setTimeout(() => cb(42), 100); }
async function getValueAsync() {
return new Promise(resolve => setTimeout(() => resolve(42), 100));
}
getValueAsync().then(v => console.log(v));Chapter Quiz — Complete all 9 topics to unlock
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