es6 数组的扩展

var arrayLike = {
  '0': 'a',
  '1': 'b',
  '2': 'c',
  'length': 3
}

var arr1 = [].slice.call(arrayLike);
console.log(arr1);  //['a','b','c']

var arr2 = Array.from(arrayLike);
console.log(arr2);  //['a','b','c']

var arr3 = Array.from('hello');
console.log(arr3);  //["h", "e", "l", "l", "o"]

let nameSet = new Set(['a','b','c','a'])
console.log(nameSet); //{"a", "b", "c"}

var arr4 = Array.from(nameSet);
console.log(arr4);  //["a", "b", "c"]

function foo(){
  var arr = Array.from(...arguments)
  console.log(arr);
}

foo([1,2,3,4,5,6])  //[1, 2, 3, 4, 5, 6]

var arr5 = Array.from({length: 3});
console.log(arr5);  //[undefined, undefined, undefined]

const toArray = (() => 
  Array.from? Array.from: obj => [].slice.call(obj) // obj => [].slice.call(obj)
)()

Array.from([1,2,3], x=>x*x);

Array.from([1,2,3]).map(x=>x*x);

Array.from([1,2,3], (x)=>x*x);

//-----------Array.of
console.log(Array.of(1,2,3)); //[1,2,3]
console.log(Array.of(3)); //[3]
console.log(Array.of(3).length);  //1
console.log(Array.of());  //[]

//copyWithin
var cw = [1,2,3,4,5,6,7,8,9].copyWithin(0, 5);  //[6,7,8,9,5,6,7,8,9]
console.log(cw);
var cw = [1,2,3,4,5,6,7,8,9].copyWithin(0,5,6); //[6,2,3,4,5,6,7,8,9]
console.log(cw);
var cw = [1,2,3,4,5,6,7,8,9].copyWithin(1,5,6); //[1,6,3,4,5,6,7,8,9]
console.log(cw);

//find() findIndex()
var arr6 = [1,2,3,4,5,6,7,8,9].find((n) => n > 3)
console.log(arr6);  //4
var arr7 = [1,2,3,4,5,6,7,8,9].find(function(n){
  return n > 5;
});
console.log(arr7);  //6 //返回第一个匹配的值 //findIndex返回第一个匹配的值的位置

//fill
var arr8 = ['a','b','c'].fill(7)
console.log(arr8); //[8,8,8]

var arr9 = [1,2,3,4,5,6,7,8,9].fill('aa' , 2,4);
console.log(arr9);  //[1,2,'aa','aa',5,6,7,8,9]

//entries keys values
for(let key of ['a','b','c'].keys()){
  console.log(key);
}
//0,1,2

for(let value of ['a','b','c'].values()){
  console.log(value);
}
//a,b,c
for(let entry of ['a','b','c'].entries()){
  console.log(entry);
}
//[0, 'a'][1, 'b'][2, 'c']

//数组includes
console.log([1,2,3].includes[2]); //true
console.log([1,2,3].includes[4]); //false

//空数组
console.log(Array(3)) //[undefined × 3] //书上说返回[,,,]但是我确返回了[undefined × 3]
console.log(0 in [undefined,undefined,undefined]) //true
console.log(0 in [,,,]);  //false

//数组推导
var a1 = [1,2,3,4];
//var a2 = [for(n of a1)n*2] 报错了了
//console.log(a2);