ReduceRight

ReduceRight is a method of the JavaScript Array prototype that applies a function to each element of an array from right to left, reducing the array to a single value. It serves as the right-to-left counterpart to reduce, which traverses from left to right.

The syntax is array.reduceRight(callback, initialValue). The callback receives four arguments: accumulator, currentValue, index, and the array itself. During the reduction, the callback returns the updated accumulator. If an initialValue is provided, it is used as the starting accumulator and the callback is invoked for every element from the last index down to the first, skipping any holes in the array. If initialValue is not provided, the last defined element becomes the initial accumulator and the function proceeds from the second-to-last element toward the first. If the array is empty and no initialValue is given, a TypeError is thrown.

Behavior with sparse arrays and holes: reduceRight skips indices that do not contain defined elements. This means holes do not trigger the callback. If there is no initialValue and the array consists only of holes, the operation results in a TypeError.

Common uses include right-associative operations and string construction from right to left. For example, [1, 2, 3].reduceRight((acc, cur) => acc - cur) yields 0, because it starts with 3 as the initial accumulator and applies 3 - 2, then the result minus 1.

Compatibility and notes: reduceRight is widely supported in modern JavaScript environments. It operates on concrete Array instances, so array-like objects should be converted first (for example, via Array.from) if reduction on such objects is needed.