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Build a Dataloader-Style Batching Utility That Coalesces Individual Async Calls

Q: Build a Dataloader-Style Batching Utility That Coalesces Individual Async Calls

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the entire real point of this pattern — that MULTIPLE separate, concurrent `load()` calls collapse into exactly ONE real bulk request — was verified directly: 3 concurrent calls produced exactly 1 real `batchFn` invocation, with results correctly matched back to their own individual callers. ## 1. The problem, restated Calling a real data-fetching function once PER individual key (`fetchUser(1)`, `fetchUser(2)`, `fetchUser(3)` as 3 separate real network requests) is genuinely wasteful when a real backend could serve all 3 in ONE bulk request. A DataLoader-style batcher lets calling code keep writing simple, individual `load(key)` calls, while transparently COALESCING every call made within the same real tick into one combined `batchFn(keys)` call behind the scenes. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | |

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The problem

Problem, as an interviewer would state it: "Build a batching utility (like Facebook's DataLoader): individual load(key) calls made within the SAME tick should coalesce into ONE real bulk request, with each caller correctly getting back only their own result."

Examples:

Code

const [u1, u2, u3] = await Promise.all([loader.load(1), loader.load(2), loader.load(3)]);
// only ONE real batchFn([1,2,3]) call happens, despite 3 separate load() calls

Clarifying questions expected:

What defines "the same tick" for batching purposes — the current synchronous execution, a microtask boundary, or a real timer-based window?
Does the batch function receive keys in the exact order they were requested, and must results map back correctly by that same order?
Should a load() call made in a LATER tick (after the current batch has already started) join the same batch, or start a new one?

Code / implementation expected: Yes — real, direct proof that 3 concurrent load() calls trigger exactly 1 real batch call, with each result correctly routed to its own original caller.

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the entire real point of this pattern — that MULTIPLE separate, concurrent load() calls collapse into exactly ONE real bulk request — was verified directly: 3 concurrent calls produced exactly 1 real batchFn invocation, with results correctly matched back to their own individual callers.

1. The problem, restated

Calling a real data-fetching function once PER individual key (fetchUser(1), fetchUser(2), fetchUser(3) as 3 separate real network requests) is genuinely wasteful when a real backend could serve all 3 in ONE bulk request. A DataLoader-style batcher lets calling code keep writing simple, individual load(key) calls, while transparently COALESCING every call made within the same real tick into one combined batchFn(keys) call behi

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Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: 3 concurrent load() calls coalesce into exactly 1 real batchFn call, with results correctly routed to their own callers

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