How do you implement a polyfill for reduce?
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.
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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.
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.
Three arguments: the current element, the current index, and the array itself. Use callback.call(thisArg, this[i], i, this) to support the optional thisArg parameter.
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