JavaScript Core interview questions

Closures, the event loop, prototypes, coercion, and the async semantics in every frontend screen.

4 questions in Frontend Engineering.

Frontend EngineeringA 22-minute read on how the topic fits together, before the questions.Frontend fundamentals57 short answers on one page, for revising rather than studying.

medium4

mediumConceptScenario

A save button calls an API, the request fails, the error appears in the console - and the UI still shows 'Saved'. Walk me through what is actually happening in that promise chain.

A catch handler that returns normally converts a rejected promise into a fulfilled one, so every then after it runs as though nothing failed. Combined with fetch, which only rejects on network errors and resolves happily on a 500, the chain reports success twice over. Fix by re-throwing from catch, checking response.ok, and modelling UI state as a machine.

4 minentry, mid, senior

mediumConceptCoding

How is `this` determined in JavaScript, and why does a method lose it when you pass it as a callback?

JavaScript `this` binding is decided by the call site: method calls bind the receiver, constructors bind the new object, bind/call/apply bind explicitly, and callbacks often lose the receiver because the method was passed as a plain function. It also connects arrow functions to the point an interviewer is testing.

4 minentry, mid, senior

mediumCodingConcept

Here is a script mixing setTimeout, a promise chain and an await. Tell me the order the logs come out in, and why.

The engine finishes the current task, then drains the whole microtask queue before taking another task, so every promise callback and await resumption runs before a zero-delay timeout, in the order each was queued. Use this event loop answer to show the decision, trade-off, and evidence rather than a memorised definition. It also connects microtasks to the point an interviewer is testing.

4 minmid, senior

mediumConceptCoding

What is a closure, and how does one end up leaking memory?

A closure is a function plus the scope it was created in, held by reference rather than copied. It leaks when something long-lived keeps the function alive, because everything the captured scope can reach stays reachable with it.

4 minentry, mid, senior