The reduce method contains two parameters, the first parameter is a callback function, which contains four parameters, the second parameter is an optional parameter, as the first parameter of the callback function. It executes the callback function for each element in the array in turn, excluding elements that are deleted or never assigned in the array, and accepts four parameters: the initial value (or the return value of the last callback function), the current element value, the current Index, the array on which reduce is called.