JavaScript Interview

A closure is a function bundled together with references to its surrounding lexical environment, giving the inner function access to the outer function's variables even after the outer function has returned.

typeof returns 'undefined', 'boolean', 'number', 'bigint', 'string', 'symbol', and 'function'. Notably typeof null returns 'object' (a historical bug).

Each object has an internal [[Prototype]] link (accessible via Object.getPrototypeOf or __proto__). When a property is not found on an object, the engine walks up the prototype chain until it finds the property or reaches null.

`prototype` is a property on constructor functions used as the [[Prototype]] of instances created with `new`. `__proto__` is an accessor on each object exposing its actual [[Prototype]] link.

A runtime mechanism that repeatedly takes the next task from the task queue (when the call stack is empty), runs it to completion, drains the microtask queue, then renders/repeats. It enables non-blocking concurrency on a single thread.

After each macrotask (e.g. setTimeout callback, I/O), the engine drains ALL microtasks (Promise callbacks, queueMicrotask, MutationObserver) before the next macrotask. So microtasks always run before the next macrotask.