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Durgesh Kumar Jha (djha11) wants to merge 2 commits into
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Clarify callback-based fetcher example #3696

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10dac39
Clarify callback-based fetcher example
djha11 Nov 9, 2026
e984ca4
Explain callback-based fetcher return type
djha11 Nov 9, 2026
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6 changes: 4 additions & 2 deletions packages/documentation/copy/en/declaration-files/Do's and Don'ts.md
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Original file line number Diff line number Diff line change
Expand Up @@ -79,7 +79,7 @@ function fn(x: () => void) {
```ts
/* WRONG */
interface Fetcher {
getObject(done: (data: unknown, elapsedTime?: number) => void): void;
fetchObject(done: (data: unknown, elapsedTime?: number) => void): void;
}
```

Expand All @@ -93,10 +93,12 @@ it's always legal to provide a callback that accepts fewer arguments.
```ts
/* OK */
interface Fetcher {
getObject(done: (data: unknown, elapsedTime: number) => void): void;
fetchObject(done: (data: unknown, elapsedTime: number) => void): void;
}
```

The method returns `void` because it delivers its result through the callback. The callback's `elapsedTime` parameter is non-optional because the callback is always called with that argument.

### Overloads and Callbacks

❌ **Don't** write separate overloads that differ only on callback arity:
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