The answer
The event loop, microtasks vs macrotasks, promise states and chaining, error handling, and async/await. Async is a huge part of modern JS, and is tested heavily in interviews.
JavaScript Fundamentals Interview Questions You Can't Skip
What JavaScript async topics must I know?
More FAQs in JavaScript Fundamentals Interview Questions You Can't Skip
let/const/var, arrow vs regular functions, closures, the event loop and async, promises and async/await, prototypes and this, and equality and coercion. These come up in almost every JS interview.
Because they are foundational and tested in almost every JS interview. Closures enable data privacy, callbacks, and patterns like once and memoize, so understanding them is essential for any non-trivial JavaScript.
Arrow functions do not have their own this or arguments, and cannot be used as constructors. They inherit this from the surrounding scope, which is why they are common in callbacks. Regular functions have their own this.
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