SOURCE

let a = [
"1","2","3","4"
];
const aKeys = a.keys();
console.log(aKeys)// [object Array Iterator]
//如果done属性的值为false，就意味着还有可迭代的值。
console.log(JSON.stringify(aKeys.next()))//{value:0,done:false}
console.log(JSON.stringify(aKeys.next()))//{value:1,done:false}
console.log(JSON.stringify(aKeys.next()))//{value:2,done:false}
console.log(JSON.stringify(aKeys.next()))//{value:3,done:false}
console.log(JSON.stringify(aKeys.next()))//{value:undefined,done:false}

const aValues = a.values();
console.log(aValues)// [object Array Iterator]
console.log(JSON.stringify(aValues.next()))//{value:"1",done:false}
console.log(JSON.stringify(aValues.next()))//{value:"2",done:false}
console.log(JSON.stringify(aValues.next()))//{value:"3",done:false}
console.log(JSON.stringify(aValues.next()))//{value:"4",done:false}
console.log(JSON.stringify(aValues.next()))//{value:undefined,done:false}