basic.md

JavaScript Interview FAQ — Basic

A compact set of common JavaScript interview questions with concise answers.

1. What is the event loop? How does it handle asynchronous callbacks?

The event loop checks the call stack; when empty it drains microtasks (promises) then runs one macrotask (e.g., setTimeout) and repeats.

2. Difference between var, let, and const

var is function-scoped and hoisted; let/const are block-scoped. const can't be reassigned.

3. Explain closures

A closure is a function that retains access to its lexical scope even when executed elsewhere — useful for private state.

4. == vs ===

== coerces types before comparing; === is strict (no coercion).

5. How this is determined in JS?

Determined by call-site: default/global, object method (implicit), explicit (call/apply), new, or lexical for arrow functions.

6. Prototype vs class

class is syntax sugar over prototype-based inheritance. Objects inherit via the prototype chain.

7. Call, apply, bind

call(thisArg, ...), apply(thisArg, [args]) invoke immediately; bind(thisArg) returns a bound function.

8. Promises vs async/await

Promises represent eventual results; async/await is syntactic sugar for working with promises.

9. Microtasks vs Macrotasks

Microtasks (promises) run after the current stack but before the next macrotask (timers, I/O).

10. Hoisting

Function and var declarations are hoisted. let/const are hoisted but in TDZ until initialized.