数组遍历方法源码实现

forEach

Array.prototype.$forEach = function (fn) {
    var _arr = this,
      len  = _arr.length,
      ctx  = arguments[1] || window,
      item;
  for (var i = 0; i < len; i ++) {
      item = _arr[i];
    fn.apply(ctx, [item, i, _arr]);
  }
}

map

Array.prototype.$map = function (fn) {
  var _arr = this,
      len  = _arr.length,
      ctx  = arguments[1] || window,
      res  = [],
      item;
  for (var i = 0; i < len; i ++) {
      item = _arr[i];
    res.push(fn.apply(ctx, [item, i, _arr]));
  }
  return res;
}
console.log([1, 2, 3].$map(item => item * 2));

filter

Array.prototype.$filter = function (fn) {
    var _arr = this,
      len  = _arr.length,
      ctx  = arguments[1] || window,
      res  = [],
      item;
  for (var i = 0; i < len; i ++) {
      item = _arr[i];
    if (fn.apply(ctx, [item, i, _arr])) {
        res.push(item);
    }
  }
  return res;
}

some

Array.prototype.$some = function (fn) {
  var _arr = this,
      len  = _arr.length,
      ctx  = arguments[1] || window,
      item;
  for (var i = 0; i < len; i ++) {
      item = _arr[i];
    if (fn.apply(ctx, [item, i, _arr])) {
        return true;
    }
  }
  return false;
}

every

Array.prototype.$every  = function (fn) {
    var _arr = this,
      len  = _arr.length,
      ctx  = arguments[1] || window,
      item;
  for (var i = 0; i < len; i ++) {
    item = _arr[i];
      if (!fn.apply(ctx, [item, i, _arr])) {
        return false;
    }
  }
  return true;
}

reduce

Array.prototype.$reduce = function (fn, initialVal) {
    var _arr = this,
      len  = _arr.length,
      ctx  = arguments[2] || window,
      item;
  for (var i = 0; i < len; i ++) {
      item = _arr[i];
    initialVal = fn.apply(ctx, [initialVal, item, i, _arr]);
  }
  return initialVal;
}