Question 1

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below was actually run — both implementations were executed against the identical real orders array and their outputs directly compared. ## 1. Why This Even Matters — A Story First Sorting a pile of mail into labeled bins by recipient is a task every office

Question 2

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every grouping result and prototype check below was actually run in Node (v21+ required; this project verified on v24.19.0). ## 1. Why This Even Matters — A Story First A grouping bucket labeled with a plain sticky note is a regular object — perfectly fine, but it si

Question 3

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every operation's real result below was actually run in Node (Baseline-shipped across all major browsers since 2024). ## 1. Why This Even Matters — A Story First Two overlapping guest lists for two different parties: union is the combined list of everyone invited to

Question 4

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript data-structure and performance interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every number shown below is **real, measured output** from actually running the benchmark in this doc on Node.js v24.19.0 — not an assumed percentage. ## 1. Why This Even Matters — A Story First Picture two guest lists for a wedding, each a

Question 5

What is the difference between `Object.groupBy()` and `Map.groupBy()` — keys, prototypes and ordering?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every grouping and coercion claim below was actually run in Node (v21+ required; verified on v24.19.0). ## 1. Why This Even Matters — A Story First Sorting mail into pigeonholes labeled only with a HANDWRITTEN NAME (a coerced string) genuinely cannot tell two differe

Question 6

How do Import Attributes (`with { type: "json" }`) work and why did they replace import assertions?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/modules interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. The dynamic import below was actually executed against a real JSON file written to disk for this verification. ## 1. Why This Even Matters — A Story First Handing a customs officer a sealed package without a label used to mean the officer had to guess, or you had to separ

Question 7

What does `using` / `await using` do for Explicit Resource Management (ES2024)?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every disposal order and error-path claim below was actually run natively in Node — this feature genuinely runs without a transpiler. ## 1. Why This Even Matters — A Story First Checking several items out of a library one at a time and being trusted to return each on

Question 8

What is `Array.fromAsync()` and how does it handle async iterables differently from `Array.from()`?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every collection result below was actually run in Node (Chrome 121+/Firefox 115+/Safari 16.4+/Node 22+). ## 1. Why This Even Matters — A Story First Handing a clerk a stack of sealed envelopes: the plain, quick clerk (`Array.from`) genuinely just stacks the sea

Question 9

What does `Promise.try()` do and why is it better than `Promise.resolve().then(fn)`?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript/async interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every throw and catch path below was actually run in Node. ## 1. Why This Even Matters — A Story First Calling a function directly and hoping any error gets funneled into a `.catch()` somewhere downstream is like shouting a warning INTO a mailbox and hoping the mail carrier

Question 10

When would you reach for Map.groupBy() instead of Object.groupBy(), and what problem does it solve with non-string keys?

Accepted Answer

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every claim below was actually run with real, distinct object keys — not asserted from documentation. ## 1. Why This Even Matters — A Story First Sorting mail into pigeonholes labeled by the SENDER'S NAME works fine when every sender's name is unique text. But if you

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Easy

How would you use Object.groupBy() to group an array of orders by status, and how does it compare to a manual reduce()?

How does Object.groupBy() work in ES2024 and when should you use it over reduce?

How do the new Set methods `union()`, `intersection()`, `difference()` and `symmetricDifference()` work?

Medium

Set methods deep-dive: performance and mutability trade-offs vs manual loops?

What is the difference between `Object.groupBy()` and `Map.groupBy()` — keys, prototypes and ordering?

How do Import Attributes (`with { type: "json" }`) work and why did they replace import assertions?

What does `using` / `await using` do for Explicit Resource Management (ES2024)?

What is `Array.fromAsync()` and how does it handle async iterables differently from `Array.from()`?

What does `Promise.try()` do and why is it better than `Promise.resolve().then(fn)`?

When would you reach for Map.groupBy() instead of Object.groupBy(), and what problem does it solve with non-string keys?

What does Promise.withResolvers() solve and how does it replace the deferred anti-pattern?

Hard

What does the RegExp `v` flag (Unicode Sets) add over the `u` flag?

How does the Temporal API fix JavaScript Date — Instant, PlainDate and ZonedDateTime?