【Q640】如何实现数组函数 reduce

[shfshanyue]

满足以下两个测试用例

// => 55
reduce([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], (x, y) => x + y)

// => 155
reduce([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], (x, y) => x + y, 100)

// => NaN
reduce([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], (x, y) => x + y, undefined)

以下有一个特殊的测试用例，可考虑，可不考虑

// 在 lodash 中为 NaN
// 在原生API 中为 15
reduce([1, 2, 3, 4, 5,,,,,,,,,,,], (x, y) => x + y)

TC39 规范在此: https://tc39.es/ecma262/#sec-array.prototype.reduce。可参考标准，但无需按照标准实现。

[shfshanyue]

代码见 如何实现数组函数 reduce，可调试与测试用例

const reduce = (list, fn, ...init) => {
  let next = init.length ? init[0] : list[0]
  for (let i = init.length ? 0 : 1; i < list.length; i++) {
    next = fn(next, list[i], i)
  }
  return next
}

[haotie1990]

Array.prototype.Reduce1 = function(callback, initialValue) {
  if (typeof callback !== 'function') {
    throw new TypeError('callback not a function');
  }

  const array = this;
  const len = array.length;
  let accumulator = null;
  let currentIndex = 0;
  let currentValue = null;
  if (initialValue == null) {
    // 没传入初始值的时候，取数组中第一个非 empty 的值为初始值
    while(currentIndex < len && !(currentIndex in array)) {
      currentIndex++;
    }
    if (currentIndex >= len) {// 未提供initialValue且无法在数组中找到有效值,报错
      throw new Error('array is empty and initialValue is null');
    }
    accumulator = array[currentIndex++];
  } else {
    accumulator = initialValue;
  }

  while (currentIndex < len) {
    if (currentIndex in array) {
      currentValue = array[currentIndex];
      accumulator = callback(accumulator, currentValue, currentIndex, array);
    }
    currentIndex++; 
  }
  return accumulator;
}

[heretic-G]

Array.prototype.reduce = function reduce (fun, init) {
    const length = this.length
    let result
    let start
    if (typeof fun !== 'function') {
        throw new TypeError('is not fun')
    }
    if (length === 0 && init === undefined) {
        throw new TypeError('')
    }
    if (init !== undefined) {
        result = init
        start = 0
    } else {
        for (let i = 0; i < length; i++) {
            if (this.hasOwnProperty(i)) {
                result = this[i]
                start = i + 1
                break
            }
        }
        if (start === undefined) {
            throw new TypeError('')
        }
    }

    for (let i = start; i < length; i++) {
        if (this.hasOwnProperty(i)) {
            result = fun(result, this[i], i, this)
        }
    }
    return result
}

[shfshanyue]

@heretic-G 有点小问题，对于第二个测试用例

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10].reduce1((x, y) => x + y, 120)
//=> 120

[Asarua]

function reduce(arr, cb, init) {
  const l = arr.length

  if (!l) {
    if (init) return init
    else throw new TypeError('Error')
  }

  if (init) {
    for (let i = 0; i < l; i++) {
      init = cb(init, arr[i], i, arr)
    }
    return init
  } else {
    let final
    for (let i = 0; i < l; i++) {
      final = cb(
        !i ? arr[i++] : final,
        !i ? arr[i++] : arr[i],
        i,
        arr
      )
    }
    return final
  }
}

[Kiera569]

 Array.prototype._reduce = function(arr, fn, defaultPre) {
  let sum = 0;
  let pre = defaultPre ?? arr[0];
  for(let i = 0; i < arr.length; i+=1) {
    pre = sum;
    sum = fn(pre, arr[i], i, arr);
  }  
  return sum;
 }

[haotie1990]

满足以下两个测试用例

// => 55
reduce([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], (x, y) => x + y)

// => 155
reduce([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], (x, y) => x + y, 100)

@shfshanyue

数组的map、forEach、filter等方法，需要考虑callback函数只会在有值的索引上被调用；那些从来没被赋过值或者使用 delete 删除的索引则不会被调用 [0, , 2 , 3]

[shfshanyue]

@haotie1990 这块确实没想到，我写一下

---

我查了下 lodash.reduce 没有此测试用例，因此我把它贴在题目描述中，可实现可不实现

[wangjs-jacky]

使用 forEach遍历可以过滤 undefined

function reduce(arr, fn, init) {
  let pre = init ? init : arr[0];
  let startIndex = init ? 0 : 1;
  arr.slice(startIndex).forEach((cur) => {
    pre = fn(pre, cur);
  });
  console.log(pre);
  return pre;
}

[caomeibuaichibaicai]

@Kiera569 如果是undefined,测试用例无法通过

[yxw007]

function reduce(handle, initial) {
    let arr = this;

    if (initial != null && arr.length == 0) {
        return initial;
    }

    if (arr.length <= 0) {
        throw new Error("array can't is empty");
    }

    function next(pre, index) {
        if (index >= arr.length) {
            return pre;
        }

        let r = handle(pre, arr[index], index, arr);
        return next(r, index + 1);
    }

    let res;
    if (initial) {
        res = next(initial, 0);
    } else {
        res = next(arr[0], 1);
    }
    return res;
}

Array.prototype.reduce = reduce;

let arr = [1, 2, 3];
let res = arr.reduce((pre, cur, index, array) => {
    return pre + cur;
}, 0);

console.log(res);

[JLUssh]

Array.prototype.Reduce1 = function(callback, initialValue) {
  if (typeof callback !== 'function') {
    throw new TypeError('callback not a function');
  }

  const array = this;
  const len = array.length;
  let accumulator = null;
  let currentIndex = 0;
  let currentValue = null;
  if (initialValue == null) {
    // 没传入初始值的时候，取数组中第一个非 empty 的值为初始值
    while(currentIndex < len && !(currentIndex in array)) {
      currentIndex++;
    }
    if (currentIndex >= len) {// 未提供initialValue且无法在数组中找到有效值,报错
      throw new Error('array is empty and initialValue is null');
    }
    accumulator = array[currentIndex++];
  } else {
    accumulator = initialValue;
  }

  while (currentIndex < len) {
    if (currentIndex in array) {
      currentValue = array[currentIndex];
      accumulator = callback(accumulator, currentValue, currentIndex, array);
    }
    currentIndex++; 
  }
  return accumulator;
}

好像不太对... 例 let arr = [1, 2, 3]; arr.reduce((a, b) => a + b, null) => 6 arr.reduce((a, b) => a + b, undefined) => NaN 要判断的是给定的形参个数，而不是其值