entries()_method.js

var arr = ['a', 'b', 'c', 'd', 'e'];
var eArr = arr.entries();

console.log(eArr.next()); // { value: [ 0, 'a' ], done: false }
console.log(eArr.next().value); // [1, 'b']

// 剩下c,d,e還沒跑出來，所以...
for (let e of eArr) {
  console.log(e);
}

// [ 2, 'c' ]
// [ 3, 'd' ]
// [ 4, 'e' ]