JavaScript Interview Questions

The call stack runs synchronously; completed async work queues callbacks. Microtasks (promises) drain fully after each task, before the next macrotask (timers, IO), which is why a promise chain runs before a setTimeout(0).

A closure captures variables from its defining scope. The classic bug is capturing a loop variable declared with var — every callback sees the final value; let creates a fresh binding per iteration.

== applies type coercion before comparing; === compares type and value. Use === unless you deliberately want null and undefined to compare equal.

Objects hold a link to a prototype object; property lookup walks that chain until it finds a match. class syntax is sugar over the same mechanism.

Debounce delays execution until activity stops — good for search input. Throttle guarantees at most one call per interval — good for scroll and resize handlers.