JS数组中（for...in）和(for...of)的区别

stackoverflow上有一个类似的问题

https://stackoverflow.com/questions/29285897/difference-between-for-in-and-for-of-statements

 //for..of与for..in的区别
    var arr = [3, 5, 7];
    arr.foo = "hello";

    for (var i in arr) {
        console.log("for in:", i); // logs "0", "1", "2", "foo"
    }

    for (var i of arr) {
        console.log("for of:", i); // logs "3", "5", "7"
        // it doesn't log "3", "5", "7", "hello"
    }

有人回复的答案如下：大概意思应该就是for in输出的是对象的属性；而for of 输出的是数组的每一个值