实现数组遍历

ES4及以前

for (var index = 0; index < myArray.length; index++)
    {console.log(myArray[index]);
}

缺点：不简洁

ES5

myArray.forEach(function (value) {
    console.log(value)
});

缺点：不能使用break语句中断循环，也不能使用return语句返回到外层函数

ES5

var myArray = ["a","b","c"];
for (var index in myArray) {
    console.log(myArray[index]);
}

运行结果

0
1
2

缺点：index拿到的不是"a","b","c",而是索引0,1,2，这其他语言不太一样

再举个🌰

ES5

Object.prototype.objCustom = function () {}; 
Array.prototype.arrCustom = function () {};
var myArray = ["a","b","c"]
myArray.testKey = "hello"
for (var index in myArray) {
    console.log(index)
}

运行结果

0
1
2
testKey
arrCustom
objCustom

缺点：for-in 循环体除了遍历数组元素外，还会便利自定义属性，就连数组原型链上的属性都能被访问到

Tips: for-in 使用范围，for-in常用于遍历对象的属性，不适合数组的遍历

ES6 for-of

var myArray = ["a","b","c"];
for (var value of myArray) {
    console.log(value);
}

运行结果

a
b
c

缺点：IE部分浏览器不兼容

优点：除了解决了以上缺点外，还支持Dom NodeList对象，也支持字符串遍历，也支持Map和Set对象遍历

ES6 遍历Set对象

var uniqueWords = new Set(words);
for (var word of uniqueWords) {
   console.log(word);
}

ES6 遍历Map，需要用到解构

for (var [key, value] of phoneBookMap) {
   console.log(key + "'s phone number is: " + value);
}

ES6 遍历对象属性

for (var key of Object.keys(someObject)) {
  console.log(key + ": " + someObject[key]);
}

Tips:for-of 更适合迭代器，而for-in更适合遍历对象属性