几个JS高阶函数的简单底层实现

October 07, 2018

高阶函数
Array.prototype.forEach
Array.prototype.map
Array.prototype.filter
Array.prototype.every
Array.protytype.some
Array.protytype.reduce

高阶函数

看了 FCC 的函数式编程章节才发现，原来 JS 里的几个高阶函数是典型的函数式编程思想的应用。不改变原值，总是返回新值以避免 side-effect。
有点好奇这些函数的底层是如何实现的，干脆练练手，全部自己尝试写一遍。暂时考虑写简化版，所以会略去异常处理部分的代码，实现全部使用 ES5 的语法。

Array.prototype.forEach

Array.prototype.simpleForEach = function(callback, thisArg) {
  var length = this.length;
  for (var i = 0; i < length; i++) {
    callback.call(thisArg, this[i], i, this);
  }
};

Array.prototype.map

Array.prototype.simpleMap = function(callback, thisArg) {
  var newArr = [];
  var length = this.length;
  for (var i = 0; i < length; i++) {
    newArr.push(callback.call(thisArg, this[i], i, this));
  }
  return newArr;
};

Array.prototype.filter

Array.prototype.simpleFilter = function(callback, thisArg) {
  var newArr = [];
  var length = this.length;
  for (var i = 0; i < length; i++) {
    if (callback.call(thisArg, this[i], i, this)) {
      newArr.push(this[i]);
    }
  }
  return newArr;
};

Array.prototype.every

Array.prototype.simpleEvery = function(callback, thisArg) {
  var length = this.length;
  for (var i = 0; i < length; i++) {
    if (!callback.call(thisArg, this[i], i, this)) {
      return false;
    }
  }
  return true;
};

Array.protytype.some

Array.prototype.simpleSome = function(callback, thisArg) {
  var length = this.length;
  for (var i = 0; i < length; i++) {
    if (callback.call(thisArg, this[i], i, this)) {
      return true;
    }
  }
  return false;
};

Array.protytype.reduce

Array.prototype.simpleReduce = function(callback, initialVal) {
  var accumulator,
    length = this.length;
  if (initialVal === undefined) {
    //无初值：将首个元素赋给accumulator，并从index=1的元素开始循环
    accumulator = this[0];
    for (var i = 1; i < length; i++) {
      accumulator = callback(accumulator, this[i], i, this);
    }
  } else {
    //有初值：将initialVal赋给accumulator，从index=0的元素开始循环
    accumulator = initialVal;
    for (var i = 0; i < length; i++) {
      accumulator = callback(accumulator, this[i], i, this);
    }
  }

  return accumulator;
};