JavaScript interview questions and answers

I define a Props interface and annotate the function parameter.

Any turns off checking, so I can access any property and hide bugs.

I model idle, loading, success with data, and error with a message as a discriminated union.

It tells you when an element enters the viewport without scroll listeners.

PushState and replaceState change the URL without a full reload, which SPAs use for routing.

All fails fast if any promise rejects, which is good when every request is required.

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.

The effect starts after paint, fetch is scheduled, and the function yields at await so React can keep working.

I reproduce by changing filters quickly and logging request ids.

I use a status field like idle, loading, success, error and narrow with a switch.

Interfaces with the same name in the same scope merge their members, which is useful for extending libraries.

When fetch uses credentials include, the server cannot reply Access-Control-Allow-Origin star.

They can serve shell HTML and assets offline, which is great for PWAs.

Synchronous logs run first.

Class Foo still has Foo.prototype and instances link to it.

In the effect I create a controller, pass signal to fetch, and abort in cleanup.