What are the differences between var, let and const?

var is function-scoped, hoisted and initialised to undefined, and can be redeclared. let and const are block-scoped and hoisted but sit in the temporal dead zone until declared. const prevents reassignment of the binding, not mutation of the object it points to. Default to const, use let when reassigning.

What is the difference between == and ===, and how do you compare objects in tests?

=== compares without type conversion; == applies type coercion first, giving surprises like 0 == '' being true. Both compare objects by reference, not content, so two identical objects are not equal. In tests use deep equality such as toEqual or toStrictEqual, and use Object.is semantics for NaN and -0.

What is a closure? Show a practical use in test code.

A closure is a function bundled with references to the variables of the scope where it was created, so it can still read and update them after that scope has returned. Closures power private state, factories, callbacks and memoisation, and explain the classic var-in-a-loop bug.

Explain the event loop, and why await inside forEach does not wait.

JavaScript runs one call stack; when it empties, the event loop drains microtasks (promise callbacks) and then runs the next macrotask (timers, I/O). forEach ignores the promise an async callback returns, so nothing awaits it. Use for...of for sequential work or Promise.all with map for parallel.

Compare Promise.all, Promise.allSettled, Promise.race and Promise.any. When would you use each in test code?

Promise.all fulfils with all results or rejects on the first failure. allSettled waits for every promise and reports each outcome. race settles with the first promise to settle, success or failure. any fulfils with the first success and rejects with an AggregateError only if all fail.

Write a retry helper with exponential backoff for a flaky async operation.

Wrap the async function in a loop that awaits it, catches errors, decides whether the error is retryable, waits base * 2^attempt plus random jitter, caps the delay, and rethrows the last error after max attempts. Retry only idempotent, transient failures, and never use retries to hide real test flakiness.

JavaScript Interview Questions for Testers

beginner questions

intermediate questions

advanced questions

Why JavaScript questions matter for SDETs

Playwright, Cypress, WebdriverIO and most API test tools in the Node.js ecosystem are written in JavaScript or TypeScript, so JavaScript interview questions for testers focus on the parts of the language that break test code. Interviewers are less interested in trivia than in whether you will write a test that silently skips an await, compares objects with ===, or leaks state between tests through a shared variable.

The topics that come up most

Async await interview questions: why await inside forEach does not wait and how the event loop orders synchronous code, microtasks (promise callbacks) and macrotasks (timers).
JavaScript promises interview questions: Promise.all vs allSettled, race and any. Promise.all rejects on the first failure, which suits setup steps that must all succeed; allSettled suits cleanup, where you want every attempt to run.
Closures and var, let and const: scope bugs, counters and factories in test helpers.
Equality: === compares object references, so two identical API payloads are not equal; tests need deep equality such as toEqual.

A worked example: a retry helper

A common live-coding task is "write a retry with exponential backoff". A good answer is short and handles the edge cases: retry only a bounded number of times, wait baseMs * 2 ** attempt between tries (optionally with jitter), and rethrow the last error so the test fails with a useful message. In code: for (let attempt = 0; attempt < retries; attempt++) { try { return await fn(); } catch (e) { lastError = e; await sleep(baseMs * 2 ** attempt); } } throw lastError;. Expect a follow-up: retries can hide real defects, so in a test suite they belong around known-unstable infrastructure, not around assertions. See the retry with backoff model answer.

What interviewers listen for in your code

Every promise is awaited or returned. A missing await before an assertion can let a test finish before the check runs, so it passes without testing anything.
The right collection for the job: Set for uniqueness, Map for lookups by key, Array.prototype.reduce or Object.groupBy for grouping test results.
Edge cases stated out loud: empty arrays, null versus undefined, duplicate values and rejected promises.
Readable names and small functions. Test helpers are read far more often than they are written, and interviewers notice clarity.

Practise by running code

Reading answers is not enough for JavaScript. The JavaScript coding challenges run your code against real unit tests in a sandboxed worker, including event loop ordering, retrying a flaky call and implementing Promise.all. If you work in TypeScript, continue with the TypeScript interview questions.