如何编写递归化简函数？

Question

I am trying to write a reduce function using recursion. I am learning recursion and its part of an exercise so Im trying to understand what is it that doesn't work in my code. Happy about any help! Ps. its suppose to start from the end of the array (so for example yeh becomes hey)

var strings = function(previous,current) {
    return previous+current;
};

function reducing(arr,start,func) {

    if (arr.length == 0) {
        return start;
    }
    else if (arr.length == 1) {
        return arr[0];
    }
    else {
        return func(start,arr[arr.length-1]) + reducing(arr.slice(1),start,func);
    }
}

reducing(['y','e','h'],'',strings)

Answer 1

This could be the problem, instead of reducing(arr.slice(1),start,func) try reducing(arr.slice(0, arr.length-1),start,func) as below:

function reducing(arr,start,func) {
  if (arr.length == 0) {
     return start;
  }
  else if (arr.length == 1) {
     return arr[0];
  }
  else {
    return func(start, arr[arr.length-1]) + reducing(arr.slice(0, arr.length -1),start,func);
 }
}

Answer 2

Another solution:

const reduce = (arr, fn, initial) =>
  (reduceAux = (index, value) =>
    index > arr.length-1
      ? value
      : reduceAux(index+1, fn(value, arr[index], index, value))
  )(0, initial);