javascript - 为什么这个 reduce working (JS) 的递归定义不是？

Question

我正在尝试使用递归函数重新定义 Javascript 的 reduce。这是我的尝试，这是行不通的。如果任何人都可以稍微改变它以使其工作，那就太好了，因为我会更好地理解它。 (这是 functional-javascript-workshop 中的练习)。

function reduce(arr, fn, initial) {
  if (arr.length) {
    var newArr = arr.slice(1, arr.length);
    return reduce(newArr, fn, fn(arr[0]));
  } else {
    return initial;
  }
}

module.exports = reduce

它给了我以下神秘的错误消息，我不知道如何解释:

/usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:13
    prev[curr] = ++prev[curr] || 1
                            ^

TypeError: Cannot create property 'undefined' on string 'exercitation'
    at /usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:13:29
    at reduce (/home/david/node-school/functional-workshop/solution.js:7:28)
    at /usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:12:10
    at obtainResult (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:100:21)
    at Exercise.<anonymous> (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:66:27)
    at next (/usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:188:19)
    at /usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:195:7
    at Exercise.<anonymous> (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:34:5)
    at next (/usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:188:19)
    at /usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:195:7

Answer 1

您需要更新累加器:

function reduce(arr, fn, acc) {
  if (arr.length) {
    var newArr = arr.slice(1, arr.length);
    acc = fn(arr[0], acc) 
    return reduce(newArr, fn, acc);
  } else {
    return acc;
  }
}

console.log(reduce([1,2,3], (val, sum) => sum + val, 0))