JavaScript Interview Questions and Answers

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.