JavaScript不清不楚之entries、keys、values

Array.prototype.entries

以下代码均来自：MDN

1. entries方法返回一个新的Array Iterator对象，每个元素项是包含数组中每个索引的键/值对
2. IE系列不支持

var arr = ["a", "b", "c"];
var iter = arr.entries();
var res = [];

for(var i=0; i< arr.length; i++){    // 注意，是length+1，比数组的长度大
    var tem = iter.next();           // 每次迭代时更新next
    console.log(tem.done);           // 遍历迭代器结束done才是true
    res[i]=tem.value;
}

console.log(res);                    //[[0, "a"], [1, "b"], [2, "c"]]

var arr = ["a", "b", "c"];
var iterator = arr.entries();

for (let e of iterator) {
    console.log(e);
}

// [0, "a"] 
// [1, "b"] 
// [2, "c"]

Array.prototype.keys

1. entries方法返回一个新的Array Iterator对象，每个元素项是数组中每个索引的键
2. IE系列不支持

//next的用法
var arr = ["a", "b", "c"];
var iterator = arr.keys();

console.log(iterator.next()); // { value: 0, done: false }
console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: undefined, done: true }

//索引迭代器会包含那些没有对应元素的索引
var arr = ["a", , "c"];
var denseKeys = [...arr.keys()];
console.log(denseKeys);  // [0, 1, 2]

Array.prototype.values

1. entries方法返回一个新的Array Iterator对象，每个元素项是数组中每个索引的值
2. IE系列不支持

let arr = ['w', 'y', 'k', 'o', 'p'];
let eArr = arr.values();
console.log(eArr.next().value); // w
console.log(eArr.next().value); // y
console.log(eArr.next().value); // k
console.log(eArr.next().value); // o
console.log(eArr.next().value); // p

//for of
let arr = ['w', 'y', 'k', 'o', 'p'];
let eArr = arr.values();
// 您的浏览器必须支持 for..of 循环
for (let letter of eArr) {
  console.log(letter);
}