JavaScript Interview Questions 2026

Tip: A closure is a function that retains access to its lexical scope even after the outer function has returned. Practical uses: data privacy (module pattern), function factories, and memoisation. Example: a counter() returning an inner function that increments a captured variable.

Tip: var is function-scoped and hoisted (initialised undefined); let and const are block-scoped with a temporal dead zone before declaration. const prevents reassignment of the binding (but object contents stay mutable). Prefer const by default, let when reassigning, and avoid var.

Tip: Synchronous code runs on the call stack. When it empties, the event loop drains all microtasks (Promise callbacks, queueMicrotask) before taking one macrotask (setTimeout, I/O). This is why a resolved Promise's .then runs before a setTimeout(0).

Tip: Objects link to a prototype via [[Prototype]] (__proto__). Property lookups walk the prototype chain until found or null. class syntax is sugar over this — constructor functions plus prototype methods. Object.create(proto) creates an object with a chosen prototype.

Tip: === is strict equality (no type coercion); == performs type coercion before comparing, leading to surprises like 0 == "" or null == undefined being true. Always use === except the deliberate x == null check for null-or-undefined.

Tip: this is determined by call site: method call → the object; plain call → undefined (strict) or global; new → the new instance. call/apply invoke with an explicit this (apply takes an args array); bind returns a new function with fixed this. Arrow functions have no own this — they capture the enclosing scope's.

Tip: A Promise represents a future value in pending/fulfilled/rejected states, chained with .then/.catch. async/await is syntactic sugar over Promises: await pauses the async function until the Promise settles, with try/catch for errors. Use Promise.all for concurrency, Promise.allSettled when failures are tolerable.

Tip: Debounce delays execution until activity stops for N ms (good for search-as-you-type, resize). Throttle runs at most once per N ms regardless of how often it is called (good for scroll, mousemove). Both limit how often an expensive handler fires.