JavaScript interview questions and answers

Var is function-scoped and can be redeclared, which surprises people.

A closure is a function that remembers variables from the scope where it was created even after that outer function has returned.

Declarations are moved to the top of their scope during compile, so you can call a function declaration before its line.

In a regular function, this depends on how it is called: object method, call or apply, or undefined in strict mode.

Double equals coerces types, so 1 equals true and empty string equals false in some cases.

Undefined means a variable was declared but never given a value, or a missing object property.

The DOM is the browser's tree of nodes built from HTML so JavaScript can read and update the page.

I select the node and call addEventListener with click and a function.

After a handler on the target runs, the event travels up through parents to document.

A Promise represents a value that will arrive later: pending, then fulfilled or rejected.

Async functions always return a Promise.

Debounce waits until the user stops firing an event for N milliseconds, then runs once.

Throttle runs at most once per interval while the event keeps firing, like scroll or mousemove.

Browsers block a page on one origin from reading responses from another origin unless the server sends Access-Control-Allow-Origin.

LocalStorage is for non-sensitive UI preferences, larger, and never sent automatically with requests.

SessionStorage is like localStorage but tied to the tab and cleared when that tab closes.

It catches wrong props, missing fields, and typo-level bugs before the browser.

Both describe object shapes

A union means a value can be one of several types, like string or number, or loading or success or error.

Fetch sends an HTTP request and returns a Promise for the Response.