JavaScript interview questions and answers

Falsy values are false, 0, empty string, null, undefined, and NaN.

Spread copies enumerable properties into a new object or array, which we use to update state immutably.

200 means success so I render the data.

It turns a JS value into a JSON string for APIs or localStorage.

It stops the browser's default action, like following a link or submitting a form.

Map returns a new array of transformed items and is the React list pattern.

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.