JavaScript Interview Questions with Answers

`var` is function-scoped, hoisted, can be redeclared. `let` is block-scoped, hoisted but not initialized (TDZ), can be reassigned. `const` is block-scoped, must be initialized at declaration, cannot be reassigned (but objects/arrays can be mutated).

A closure is a function that remembers variables from its outer scope even after the outer function has returned. Example: `function counter() { let count = 0; return () => ++count; } const inc = counter(); inc(); // 1, inc(); // 2` — The inner function "closes over" `count`.

JavaScript is single-threaded. The event loop handles async operations: (1) Call stack executes synchronous code, (2) Web APIs handle async tasks (setTimeout, fetch), (3) Callback queue holds completed callbacks, (4) Microtask queue (Promises) has higher priority, (5) Event loop moves callbacks to call stack when it's empty.

`==` (loose equality) performs type coercion before comparison: `"5" == 5` is true. `===` (strict equality) checks both value and type: `"5" === 5` is false. Best practice: always use `===` to avoid unexpected bugs.

A Promise represents a future value with 3 states: pending, fulfilled, rejected. `async/await` is syntactic sugar over Promises. `async function getData() { try { const res = await fetch(url); const data = await res.json(); return data; } catch(err) { console.error(err); } }` — Makes async code look synchronous and easier to read.

Every JS object has a `__proto__` linking to its prototype. When accessing a property, JS looks up the prototype chain. `class Dog extends Animal {}` — Dog.prototype.__proto__ === Animal.prototype. This is how methods are shared without copying.

Debouncing: delays execution until user stops triggering (e.g., search input — wait 300ms after last keystroke). Throttling: limits execution to once per interval (e.g., scroll handler — max once per 100ms). Debounce for input, throttle for scroll/resize.

Functions that take other functions as arguments OR return functions. Examples: `map`, `filter`, `reduce`, `forEach`. Custom: `const multiply = (x) => (y) => x * y; const double = multiply(2); double(5); // 10`