1 Array.every(x=>x)是每一個都要滿足
 2 
 3 Array.some(x=>x)是有一個滿足。
 4 
 5 Array.find（findIndex)，返回符合條件的第一個值。
 6 
 7 Array.filter（過濾成新的陣列）
 8 
 9 陣列的方法分為兩類
10 
11 1）改變原陣列
12 
13 push,pop,shift,unshift,sort,reverse,splice
14 
15 2）不改變原陣列concat,join-->
16 
17 split，toStringpush：從陣列最後一位開始加資料
18 
19 pop:把陣列最後一位剪下
 
20 
21 shift:在陣列最前一位剪下
22 
23 unshift:在陣列最前一位加數
24 
25 reverse:把原陣列逆轉
26 
27 splice:arr.splice(從第幾位開始，擷取多少長度，在切口處新增新資料)
28 
29 concat ：連線join:返回字串
30 
31 slice:擷取arr.slice(從該為開始擷取，擷取到該為)

示例

1.建立一個數組，判斷陣列中是否存在某個值
var newarr = [
 { num: 1, val: 'ceshi', flag: 'aa' },
 { num: 2, val: 'ceshi2', flag: 'aa2' }
]
console.log(newarr.filter(item 
 
=> item.num===2 ))
2.也可以通過上面方法過濾掉num為2的留下num為1的
var newarr = [
 { num: 1, val: 'ceshi', flag: 'aa' },
 { num: 2, val: 'ceshi2', flag: 'aa2' }
]
console.log(newarr.filter(item => item.num!=2 ))

3.去掉空陣列空字串、undefined、null
var arr = ['1','2',undefined, '3.jpg',undefined]
var newArr = arr.filter(item => item)
console.log(newArr)
 
 
var arr = ['1','2',null, '3.jpg',null]
var newArr = arr.filter(item => item)
console.log(newArr)
 >//空字串裡面不能包含空格
var arr = ['1','2','', '3.jpg','']
var newArr = arr.filter(item => item)
console.log(newArr)
4.去掉陣列中不符合項
var arr = [20,30,50, 96,50]
var newArr = arr.filter(item => item>40) 
console.log(newArr)

5.過濾不符合項
var arr = ['10','12','23','44','42']
var newArr = arr.filter(item => item.indexOf('2')<0) 
console.log(newArr)

6.陣列去重
var arr = [1, 2, 2, 3, 4, 5, 5, 6, 7, 7,8,8,0,8,6,3,4,56,2];
var arr2 = arr.filter((x, index,self)=>self.indexOf(x)===index) 
console.log(arr2); //[1, 2, 3, 4, 5, 6, 7, 8, 0, 56]

/*
7  有一個物件陣列 a ,將a數中物件某個屬性的值儲存到B陣列中
*/
var porducts = [
  {name:"cucumber",type:"vegetable"},
  {name:"banana",type:"fruit"},
  {name:"celery",type:"vegetable"},
  {name:"orange",type:"fruit"},
];
// es5
var filteredProducts = [];
for(var i = 0;i < porducts.length; i ++){
  if(porducts[i].type === "fruit"){
    // 如果條件滿足就把當前的值推入
    filteredProducts.push(porducts[i])
  }
}
// console.log(filteredProducts)//0: {name: "banana", type: "fruit"}1: {name: "orange", type: "fruit"}length: 2__proto__: Array(0)
// ES6
 var filter2 = porducts.filter(function(porduct){//對porducts陣列物件過濾如果porduct.type === "fruit"就return出去,再用一個變數接住
  return porduct.type === "fruit"
})
console.log(filter2)


/*
  需求二
 8 有一個物件陣列A,過濾掉不滿足以下條件物件
  條件:蔬菜 數量大於0 價格小於10
*/
var products = [
  {name:"cucumber",type:"vegetable",quantity:0,price:1},
  {name:"banana",type:"fruit",quantity:10,price:16},
  {name:"celery",type:"vegetable",quantity:30,price:8},
  {name:"orange",type:"fruit",quantity:3,price:6},
];
products = products.filter(function(product){
  return product.type === "vegetable"
  && product.quantity > 0
  && product.price < 10
})
console.log(products)//0: {name: "celery", type: "vegetable", quantity: 30, price: 8}name: "celery"price: 8quantity: 30type: "vegetable"__proto__: Objectlength: 1__proto__: Array(0)

/*
  需求三:
 9 有兩個陣列A,B,根據A中的ID值 ,過濾掉B陣列不符合的資料
*/
var post = {id:4,title:"javascript"};
var comments = [
  {postId:4,content:'Angular4'},
  {postId:2,content:'VUE.js'},
  {postId:3,content:'Node.js'},
  {postId:4,content:'React.js'},
];
function commentsForPost(post,comments){
  return comments.filter(function(comment){
    return comment.postId === post.id;
  })
}
console.log(commentsForPost(post,comments))
// 0: {postId: 4, content: "Angular4"}1: {postId: 4, content: "React.js"}length: 2__