1. for v in obj/array , works on both obj(like a map) and array:

for obj:

var user = {
    first_name: "CSS",
    last_name: "HTML",
    age: 4,
    /*from www.w3cschool.cn*/
    website: "www.w3cschool.cn"
};

> for(v in user) console.log(v);
first_name
last_name
age
website

> for(v in user) console.log(user[v]);
CSS
HTML
4
www.w3cschool.cn

//obj does not allow `for of` statement
> for(v of user) console.log(v);
TypeError: user is not iterable

for array:

> for(v of arr) console.log(v);
10
9
8

> arr=[10,9,8]
[ 10, 9, 8 ]
> for(v in arr) console.log(arr[v]);
10
9
8

//`for of` works in array since array can be iterated
> for(v of arr) console.log(v);
10
9
8