for ... in ... use statement

for ... in ... or for traversing an object property array;
traverse the object:

var dog = {};
dog.name = '小白'
dog.age = 3
dog.love = '骨头'
for(item in dog) {
    console.log(item) //'name','age','love'
    console.log(dog[item]) //'小白',3,'骨头'
}

Through the array:

var items = new Array()
items[0] = '小白'
items[1] = '小黑'
items[2] = '小黄'
for(i in items) {
    console.log(i) // 0,1,2
    console.log(items[i]) //'小白','小黑','小黄'
}