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Starter#Arrays#Polyfill
Array.prototype.reduce Polyfill
Implement Array.prototype.reduce from scratch, including the tricky no-initial-value path and the empty-array error case.
By modernDev2026-06-05Medium
reduce is the hardest of the three array iterators to polyfill correctly. When no initial value is provided, the first element becomes the accumulator and iteration starts at index 1. Calling reduce on an empty array with no initial value must throw a TypeError. The subtle bug most candidates write: checking initialValue === undefined instead of arguments.length >= 2.
Goal: Implement myReduce with correct handling of the missing-initialValue path and the empty-array error.
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