JavaScript & TypeScriptInterview questions and answers

An execution context is the environment where a piece of code runs. It holds the variable environment (declared vars/functions), the scope chain (which outer scopes are visible), and the value of 'this'. JS creates a new context for the global code, every function call, and every eval invocation.

The call stack is a LIFO (last-in, first-out) stack of execution contexts. When you call a function, a frame is pushed; when it returns, the frame is popped. If frames pile up faster than they pop (e.g. unbounded recursion), you get a 'Maximum call stack exceeded' error.

The event loop is the mechanism that takes callbacks from the task and microtask queues and pushes them onto the call stack — but only when the stack is empty. This is how single-threaded JavaScript handles async work without blocking: heavy callbacks wait their turn instead of running mid-stack.

Microtasks are queued by promises, queueMicrotask, and MutationObserver. Macrotasks come from setTimeout, setInterval, I/O, and UI events. All pending microtasks drain fully BEFORE the next macrotask runs — which is why promise callbacks always run before a setTimeout(…, 0).

A closure is a function that retains access to variables from its outer (lexical) scope even after the outer function has returned. Example: function outer(){ let n=0; return ()=>++n }. Each call to outer() creates a fresh n, captured by the inner function — closures power private state, currying, and memoization.