javascript - ReduceRight而不反转数组

我写了减少JavaScript的本机实现。但是我正在寻找是否可以使用此reduce来实现ReduceRight。

Array.prototype.myReduce = function(cb, initialVal) {
  if (!cb)
    throw new Error("No CB defined");

  let [accumulator, ...arr] = initialVal === undefined ? [...this] : [initialVal, ...this];

  for (var i = 0; i < this.length; i++) {
    accumulator = cb.call(undefined, accumulator, arr[i], i, arr);
  }

  return accumulator;
}

我从this article读到


  现在您可能会注意到一些事情。不只是地图的一种特殊情况
  减少？是!实际上，我们可以根据reduce的表亲来实现map
  reduceRight。 reduceRight就像reduce一样，只不过它减少了
  反向列出项目。因此，它将首先遇到Nil，
  然后是倒数第二个项目，继续前进直到到达第一个项目为止
  列表中的项目。 reduceRight可以实现如下：


我无法理解如何实现reduceRight（无需反转数组）

这是我的实现：

Array.prototype.myReduceRight2 = function(cb, initialVal) {
  if (!cb)
    throw new Error("No CB defined");

  const arr = [...this].reverse(); //DONOT REVERSE! Looking for alternate solution(s)

  const res = arr.myReduce((acc, val) => {
    return acc.concat(val);
  }, initialVal); // pass initialVal

  return res;
}

最佳答案

以下是我在个人库中使用的reduceRight的实现。

码：

Array.prototype.myReduceRight = function(callback, initialValue) {
  var
    /* Cache the length of the context. */
    length = this.length >>> 0,

    /* Create a counter defaulting at the index of the last element in the context. */
    counter = length - 1;

  /* Check whether a second argument has been given or not. */
  if (arguments.length < 2) {
    /* Repeat until the counter is less than or equal to the length of the
       context or when the counter exists as an index in the context. */
    while (counter >= 0 && !(counter in this)) {
      /* Decrement the counter. */
      counter--;
    }

    /* Set the inital value. */
    initialValue = this[counter--];
  }

  /* Repeat until the counter is less than 0. */
  while (counter >= 0) {
    /* Check whether the counter exists as an index in the context. */
    if (counter in this) {
      /* Set the initial value to be the return value of the given callback. */
      initialValue = callback.call(this, initialValue, this[counter], counter, this);
    }

    /* Decrement the counter. */
    counter--;
  }

  /* Return the calculated value. */
  return initialValue;
}

const array = [1, 2, 3, 4];
console.log(array.myReduceRight((acc, cur) => acc + cur));

关于javascript - ReduceRight而不反转数组，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/49582290/