-- Implements Javascript's `Array.prototype.reduce` as defined below
-- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce
-- TODO Luau: when Luau supports overloads, use them here so that reduceFn can correctly use T when initialValue (U) isn't supplied

return function<T, U>(array: Array<T>, callback: reduceFn<T, U>, initialValue: U?): U
	if _G.__DEV__ then
		if typeof(array) ~= "table" then
			error(string.format("Array.reduce called on %s", typeof(array)))
		end
		if typeof(callback) ~= "function" then
			error("callback is not a function")
		end
	end

	local length = #array

	local value: T | U
	local initial = 1

	if initialValue ~= nil then
		value = initialValue
	else
		initial = 2
		if length == 0 then
			error("reduce of empty array with no initial value")
		end
		value = array[1]
	end

	for i = initial, length do
		value = callback(value :: U, array[i], i, array)
	end

	return value :: U
end
