16. 数组的拓展方法

1. keys

// 例子 16-1
 
var arr = ["a", , "c"];
 
var sparseKeys = Object.keys(arr);
console.log(sparseKeys); // ['0', '2']
 
var denseKeys = [...arr.keys()];
console.log(denseKeys);  // [0, 1, 2]

2. entries

// 例子 16-2
 
var arr = ["a", "b", "c"];
var iterator = arr.entries();
 
for (let e of iterator) {
    console.log(e);
}

3. values

// 例子 16-3
 
let arr = ['w', 'y', 'k', 'o', 'p'];
let eArr = arr.values();
 
for (let letter of eArr) {
  console.log(letter);
}

4. includes

// 例子 16-4
 
// bad
function test(fruit) {
  if (fruit == 'apple' || fruit == 'strawberry') {
    console.log('red');
  }
}
 
// good
function test(fruit) {
  const redFruits = ['apple', 'strawberry', 'cherry', 'cranberries'];
  if (redFruits.includes(fruit)) {
    console.log('red');
  }
}

5. find

// 例子 16-5
 
var inventory = [
    {name: 'apples', quantity: 2},
    {name: 'bananas', quantity: 0},
    {name: 'cherries', quantity: 5}
];
 
function findCherries(fruit) {
    return fruit.name === 'cherries';
}
 
console.log(inventory.find(findCherries)); // { name: 'cherries', quantity: 5 }

6. findIndex

// 例子 16-6
 
function isPrime(element, index, array) {
  var start = 2;
  while (start <= Math.sqrt(element)) {
    if (element % start++ < 1) {
      return false;
    }
  }
  return element > 1;
}
 
console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found
console.log([4, 6, 7, 12].findIndex(isPrime)); // 2

更多的就不列举了。