What types does the main pLimit function expect for its concurrency argument?

The pLimit function accepts a concurrency parameter typed as number | Options [index.d.ts:L89-L90].

Breaking this down:

• number — a plain concurrency limit, minimum 1 [index.d.ts:L89-L90], [readme.md:L33-L45].
• Options — an object with:
  • concurrency: number (required, minimum 1) [index.d.ts:L92-L97]
  • rejectOnClear?: boolean (optional, default false) — rejects pending promises with an AbortError when clearQueue() is called [index.d.ts:L99-L106].

At runtime, index.js implements this union manually: if typeof concurrency === 'object', it destructures {concurrency, rejectOnClear = false} from it [index.js:L3-L11]. It then validates the resolved concurrency value via validateConcurrency, which throws a TypeError unless the value is a positive integer or Number.POSITIVE_INFINITY [index.js:L123-L128], [index.js:L9]. It also throws a TypeError if rejectOnClear isn't a boolean [index.js:L12-L14].

This matches the test suite, which confirms pLimit throws on 0, -1, 1.2, undefined, true, {}, and {concurrency: 0} [test.js:L275-L303].

Note that Number.POSITIVE_INFINITY is valid at runtime per validateConcurrency [index.js:L124], even though the TypeScript type just says number — infinity is a legitimate number value in JS/TS, so this is consistent with the type but worth knowing as an effectively "unlimited concurrency" option.

FOLLOW_UPS:

• How does validateConcurrency distinguish valid numbers from invalid ones?
• What happens internally when limit.concurrency is changed at runtime?
• How does rejectOnClear affect pending promises in clearQueue()?