Why does the reduce polyfill throw TypeError for empty arrays?
Because without an initial value, reduce uses the first element as the accumulator. An empty array has no first element. The spec requires: TypeError: Reduce of empty array with no initial value.
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Loop through the array, call the callback with (element, index, array), push the result to a new array, return it. Use let in the loop and do not mutate the original.
Handle the initial value: if provided, start with it at index 0; if not, use the first element at index 1. Throw TypeError for empty arrays without initial value. Loop and update the accumulator.
Return a new Promise. Track completed count. For each promise, resolve and store the result at its index. When all complete, resolve with the results array. If any rejects, reject immediately.
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