let arrayLike = {
  '0': 'a',
  '1': 'b',
  '2': 'c',
  length: 3
};
// ES5的写法
var arr1 = [].slice.call(arrayLike);
console.log(arr1) // ['a', 'b', 'c']
// ES6的写法
let arr2 = Array.from(arrayLike);
console.log(arr2) // ['a', 'b', 'c']
Array.from('hello')// ['h', 'e', 'l', 'l', 'o']
let namesSet = new Set(['a', 'b'])
Array.from(namesSet) // ['a', 'b']

Array.from({ length: 3 });//扩展运算符就转换不了
// [ undefined, undefined, undefined ]

var a = Array.from([1, 2, 3], x => x * x);
console.log(a)
// 等同于
var a1 = Array.from([1, 2, 3]).map(x => x * x);
console.log(a1)
var a2 = Array.from([1, 2, 3], (x) => x * x)
console.log(a2)// [1, 4, 9]
//只要有一个原始的数据结构，你就可以先对它的值进行处理，
// 然后转成规范的数组结构，进而就可以使用数量众多的数组方法。
var a3 = Array.from({ length: 2 }, () => 'jack')
console.log(a3)// ['jack', 'jack']

console.log(Array()) // []
console.log(Array(3)) // [, , ,]
console.log(Array(3, 11, 8)) // [3, 11, 8]
console.log(Array.of(3, 11, 8))// [3,11,8]
console.log(Array.of(3)) // [3]
console.log(Array.of()) // []

var a = [1, 4, -5, 10].find((n) => n < 0)
console.log(a)
//find方法的回调函数可以接受三个参数，依次为当前的值、当前的位置和原数组。
[1, 5, 10, 15].find(function(value, index, arr) {
  return value > 9;
}) // 10

[1, 5, 10, 15].findIndex(function(value, index, arr) {
 return value > 9;
}) // 2

['a', 'b', 'c'].fill(7) // [7, 7, 7]
new Array(3).fill(7) // [7, 7, 7]

['a', 'b', 'c'].fill(7, 1, 2)
// ['a', 7, 'c']

for (let index of ['a', 'b'].keys()) {
 console.log(index);
}
// 0
// 1
for (let elem of ['a', 'b'].values()) {
 console.log(elem);
}
// 'a'
// 'b'
for (let [index, elem] of ['a', 'b'].entries()) {
 console.log(index, elem);
}
// 0 "a"
// 1 "b"

let letter = ['a', 'b', 'c'];
let entries = letter.entries();
console.log(entries.next().value); // [0, 'a']
console.log(entries.next().value); // [1, 'b']
console.log(entries.next().value); // [2, 'c']

[1, 2, 3].includes(2);   // true
[1, 2, 3].includes(4);   // false
[1, 2, NaN].includes(NaN); // true

Array(3) // [, , ,]

console.log(Array.from(['a',,'b']))//[ 'a', undefined, 'b' ]
console.log([...['a',,'b']])// [ "a", undefined, "b" ]
let arr = [, 'a',,];
for (let i of arr) {
  console.log(i);
}//undefined a undefined

// entries()
[...[,'a'].entries()] // [[0,undefined], [1,"a"]]
// keys()
[...[,'a'].keys()] // [0,1]
// values()
[...[,'a'].values()] // [undefined,"a"]
// find()
[,'a'].find(x => true) // undefined
// findIndex()
[,'a'].findIndex(x => true) // 0