JavaScript interview questions and answers

The question mark dot stops the lookup and returns undefined if the left side is null or undefined.

Document.cookie can set cookies the page can also read, so XSS can steal them.

If I add a window resize listener in useEffect and forget the cleanup, the handler keeps the component closure alive after unmount.

A generic is a type parameter so a function or component works with many shapes while keeping the relationship.

Slice returns a shallow copy of a portion and does not change the original, which is what I want in React.

JavaScript runs on one call stack.

Destructuring pulls values from arrays or objects into named variables in one line.

Spread copies array or object properties into a new literal.

It is the browser steps from HTML and CSS to first painted pixels: parse DOM, parse CSSOM, render tree, layout, paint.

Promise callbacks are microtasks and run before the next macrotask like setTimeout.