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fix(map/set): treat nullish initialValue as a real accumulator - #2196
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Map.reduce, Set.reduce, and reduceAsync treated null and undefined as a missing initial value. An explicit null seed was dropped, and a callback that returned null skipped the next Map or Set entry. Co-authored-by: Rahul Kashyap <itisrmk@users.noreply.github.com>
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reduceAsync was already fixed in toss#2176, so only the Map and Set versions remain. Take the first entry from the iterator when no initial value is given and let the loop run the callback unconditionally. Rewrite the tests to check the callback arguments.
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Summary
Map.reduceandSet.reducetreatednull/undefinedas a missing initial value. Passingnullas the seed dropped it, and a callback that returnednullskipped the next entry. (reduceAsynchad the same bug and was already fixed in #2176.)Fix
Only omit the initial value when the argument count says it was not passed, like
es-toolkit/compat'sreduce(#1742). When no initial value is given, the first entry is taken from the iterator and the loop runs the callback unconditionally. Explicitnull/undefinedseeds and nullish callback results are kept.Test plan
yarn vitest run src/map/reduce src/set/reduce(43 passed; the 12 new tests fail on the previous implementation)yarn lintandtsc --noEmit