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JavaScript Advanced: Code That Holds Up in Production
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JavaScript Advanced: Code That Holds Up in Production

Final Check: JavaScript Advanced

Fifteen questions across the whole course, from closures to memory leaks. If you score below 12, go back to the sections where you missed and redo their Check Yourself quizzes.

15 min JavaScriptTesting and DebuggingPerformance OptimizationProgramming PatternsPackaging and Dependencies Advanced
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1. function makeGreeter(greeting) { return name => `${greeting}, ${name}!`; } Why can the returned function still use greeting after makeGreeter has returned?
greeting was copied into a global variable. The returned function is a closure that keeps a reference to makeGreeter's scope. JavaScript never frees function variables.
2. An object literal has label: () => this.name. Why does shop.label() return undefined?
Arrow functions can't return strings. this.name is private. Arrow functions take this from where they were written, not from the object they're on.
3. async function save() { console.log(1); await null; console.log(3); } save(); console.log(2); What prints?
1, 2, 3 1, 3, 2 2, 1, 3
4. Why can a promise chain that keeps queueing microtasks freeze a page?
Microtasks run on a separate thread that blocks the main one. Every microtask runs before the next task, so timers, clicks and repaints never get a turn. Promises use more memory than timers.
5. You need a result from any one of three mirror servers, whichever answers successfully first. Which do you use?
Promise.all Promise.race Promise.any
6. fetch(url, { signal: AbortSignal.timeout(5000) }) rejects after 5 seconds. What is error.name?
AbortError TimeoutError NetworkError
7. The user clicks Cancel on an import that already saved 300 of 1,000 rows. After abort(), what happened to the 300 rows?
They're rolled back automatically. They're still saved. Use a transaction if you need all or nothing. They're deleted when the AbortError is thrown.
8. A function needs to process 400,000 API records, page by page, and might stop early. What fits best?
Load every page into an array first, then loop. An async generator that fetches each page when needed, consumed with for await. Promise.all on all page numbers.
9. What is backpressure in Node streams?
A way to compress data while it streams. The writable side signalling it's full, so the reader pauses until it catches up. An error thrown when a file is too large.
10. In a JSDoc-typed file, which line describes an optional number parameter that defaults to 20?
@param {number} limit=20 @param {number?} limit @param {number} [limit=20]
11. Why prefer // @ts-expect-error over // @ts-ignore?
It becomes an error itself once the problem is fixed, so it can't stay around forever. It also silences ESLint. It fixes the error automatically.
12. After running eslint --suppress-all, a teammate adds a new file with an undefined variable. What happens in CI?
It passes, because all errors are suppressed. It fails on the new file. Old problems are tolerated, new ones block. ESLint rewrites the suppressions file to include it.
13. Which package.json field limits what other projects can import from your package?
main files exports
14. A report takes 300 ms. The profiler shows the time is in a function that filters the whole order list once per order. What's the fix?
Replace filter with a for loop. Group the orders once into a Map, then look up each customer. Run it in a setTimeout so it's async.
15. A class adds a resize listener with a closure that refers to this. close() doesn't remove it. Why is this a leak?
Closures are never garbage collected. Resize listeners use extra memory. The window keeps the listener, the listener's closure keeps the instance, so no instance is ever freed.

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Fifteen questions across the whole course, from closures to memory leaks. If you score below 12, go back to the sections where you missed and redo their Check Yourself quizzes.

Duration
15 min
Last update
08/23/2026
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