JavaScript Interview Questions and Answers
JavaScript is the first round at almost every frontend interview. Here are the questions that come up again and again, answered clearly.
What is the difference between var, let and const?
var is function-scoped and hoisted as undefined; let and const are block-scoped with a temporal dead zone. const can't be reassigned (though object contents stay mutable). Default to const, use let only to reassign, never var.
What is a closure?
A function bundled with the variables from the scope where it was defined — it keeps a live reference to them even after that scope returns. Closures power private state, memoization and debounce/throttle.
How is the value of `this` determined?
By HOW a function is called: new binding → the new object; call/apply/bind → the given object; obj.method() → obj; a plain fn() → undefined (strict) or global. Arrow functions capture `this` lexically.
Explain the event loop.
JS is single-threaded with one call stack. Async work is offloaded to host APIs; when the stack is empty the event loop runs queued callbacks. Microtasks (promises) drain fully before the next macrotask (setTimeout).
What's the difference between == and ===?
=== compares type and value with no coercion; == coerces first, producing surprises like 0 == '' being true. Always use ===, with the one exception of x == null to catch null or undefined.
What is hoisting and the temporal dead zone?
Declarations are processed before code runs. var is hoisted as undefined and functions are callable early; let/const are hoisted but stay in the TDZ until their line, so touching them early throws.
Get all 80+ JavaScript questions
This is a sample. The JavaScript Interview Kit has 80+ Q&A, coding patterns, polyfills and output traps — with structured answers.
⚡ Get the JavaScript Interview Kit → ₹299Frequently asked questions
- Do I need DSA for a JavaScript frontend role?
- Light DSA plus a few patterns (frequency counter, two pointers, sliding window, recursion) covers most frontend rounds.
- What are the most important JS topics?
- Scope and closures, this, prototypes, the event loop, promises/async, coercion and array methods — they account for most first-round questions.