SOURCE

const arr=[1,2,3];
const set=new Set(['s','b','c']);
const map=new Map([['a',1]]);

const itArr=arr[Symbol.iterator]();
const itSet=set[Symbol.iterator]();
const itMap=map[Symbol.iterator]();

console.log(itArr);
console.log(itSet);
console.log(itMap);

console.log(itSet.next());
console.log(itSet.next());
console.log(itSet.next());
console.log(itSet.next());

const obj={};  //原生的对象数据结构不具有Symbol.iterator;

console.log(obj[Symbol.iterator]);  //undefined