ES2015箭头函数与普通函数对比理解

直接返回表达式

var odds = evens.map(v => v + 1);
var nums = evens.map((v, i) => v + i);

var odds = evens.map(function (v) {
  return v + 1;
});
var nums = evens.map(function (v, i) {
  return v + i;
});

包含语句

nums.forEach(v => {
  if (v % 5 === 0)
    fives.push(v);
});

nums.forEach(function (v) {
  if (v % 5 === 0) fives.push(v);
});

包含this

不用修正箭头函数的this，this自动指向所在的上下文对象

var bob = {
  _name: "Bob",
  _friends: [],
  printFriends() {
    this._friends.forEach(f =>
      console.log(this._name + " knows " + f));
  }
};

对比非箭头函数的写法会更容易理解一些：

var bob = {
  _name: "Bob",
  _friends: [],
  printFriends: function printFriends() {
    var _this = this; // 需要修正this指向

    this._friends.forEach(function (f) {
      return console.log(_this._name + " knows " + f);
    });
  }
};