JavaScript interview questions and answers

Synchronous code runs on the call stack until it is empty.

Microtasks include Promise callbacks and queueMicrotask and they flush completely before the next macrotask.

Objects have an internal prototype link used when a property is missing.

It is the walk from an object to Object.prototype and then null while looking up a property.

I keep a timeout id, clear it on every call, and set a new timeout that fires the original function after wait ms.

Call and apply invoke the function now with a chosen this

I put one listener on a parent and use event.target to see which child was clicked.

StructuredClone is the modern built-in for many values.

Let and const exist in the scope from the start of the block but cannot be read until the declaration line.

Listeners can run on the way down from window to the target, then on the way up.

Cookies are small, sent with matching requests, and can be HttpOnly so JS cannot steal them.

A non-simple request triggers OPTIONS first.

A page can freely read data only from the same scheme, host, and port.

I pick interface for object shapes that might be extended or merged in declaration files.

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.