Array.prototype.entries

ES6 新增的内置方法。

const arr = [1, 2, 3, 4, 5];

const it = arr.entries();
console.log(it); // 数组的迭代器对象

console.log(it.next()); // {value: [0, 1], done: false}
console.log(it.next()); // {value: [1, 2], done: false}
console.log(it.next()); // {value: [2, 3], done: false}
console.log(it.next()); // {value: [4, 4], done: false}
console.log(it.next()); // {value: [5, 5], done: true}

// 第一项为下标，第二项为数组元素

var o = {
  a: 1,
  b: 2,
  c: 3
}

for (let k in o) {
  console.log(k, o[k]);
  // a 1
  // b 2
  // c 3
}

const arr = [1, 2, 3, 4, 5];

for (let v of arr) {
  console.log(v);
  // 1
  // 2
  // 3
  // 4
  // 5
}

const arr = [1, 2, 3, 4, 5];

const it = arr.entries();

for (let v of it) {
  console.log(v);
  // [0, 1]
  // [1, 2]
  // [2, 3]
  // [3, 4]
  // [4, 5]
}

for (let [i, v] of it) {
  console.log(i, v);
  // 0 1
  // 1 2
  // 2 3
  // 3 4
  // 4 5
}

数组实际上就是一个特殊的对象，key 键名是一个从 0 开始有序递增的数字，按顺序对应数组的每一个元素。

// 类数组

const o = {
  0: 1,
  1: 2,
  2: 3,
  length: 3
}

// o is not iterable
for (let v of o) {
  console.log(v);
}

const o = {
  0: 1,
  1: 2,
  2: 3,
  [Symbol.iterator]: Array.prototype[Symbol.iterator],
  length: 3
}

for (let v of o) {
  console.log(v);
  // 1
  // 2
  // 3
}

Object.prototype[Symbol.iterator] = Array.prototype[Symbol.iterator];

const o = {
  0: 1,
  1: 2,
  2: 3,
  length: 3
}

for (let v of o) {
  console.log(v);
  // 1
  // 2
  // 3
}

const o = {
  0: 1,
  1: 2,
  2: 3,
  length: 3
}

for (let v of Array.from(o)) {
  console.log(v);
  // 1
  // 2
  // 3
}

Array.from 也可以转换类数组为数组。

var newArr = [];

const arr = [1, 2, 3, 4, 5];

const it = arr.entries();

for (var i = 0; i < arr.length + 1; i++) {
  var item = it.next();
  
  !item.done && (newArr[i] = item.value);
}

console.log(newArr); // [[0,1],[1,2],[2,3],[3,4],[4,5]]

二维数组排序

const newArr = [
  [56, 23],
  [56, 34, 100, 1],
  [123, 234, 12]
];

function sortArr (arr) {
  const _it = arr.entries();
  let _doNext = true;
  	
  while (_doNext) {
    const _r = _it.next();
    
    if (!_r.done) {
      _r.value[1].sort((a, b) => a - b);
      _doNext = true;
    } else {
      _doNext = false;
    }
  }
  
  return arr;
}

console.log(sortArr(newArr));
// [[23,56],[1,34,56,100],[12,123,234]]