JavaScript Interview Prep Flashcards

What is a closure in JavaScript?

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.

What are the three primitive value types of `typeof` that are NOT object?

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

How does prototypal inheritance work in JavaScript?

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.

Difference between `__proto__` and `prototype`?

`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.

What is the JavaScript event loop?

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.