6.数组

1-1 二维数组

       // 二维数组  数组里面套数组
        var arr=[[1,2],[3,4]];
        console.log(arr[0][1])//2
        console.log(arr[1][0])//3

1-2 增

push();
unshift();
concat();
splice(index,0,...value) //从某个下标的前面增加

     // 1.可以添加一个值，可以添加多个值
        // 2.不能拼接数组
      var arr=[1,2,3,4];
        // var b=[5,6,7];
        arr.push(5);
        console.log(arr);//1 2 3 4 5

                 var arr=[1,2,3,4];
        arr.unshift(0);
        console.log(arr);//0,1,2,3,4
        var a=[1,2,3];
        var b=[4,5,6];
        b.unshift(...a);//向前添加数组需要展开
        console.log(b);//1,2,3,4,5,

1-3 展开数组

                 // ...arr  可以将一个数组展开
        var arr=[1,2,3];
        console.log(arr);//1,2,3
        console.log(...arr);//
        var a=[1,2,3];
        var b=[4,5,6];
        a.push(...b);
        console.log(a);//1,2,3,4,5,6

1-5 concat

                 var arr =[1,2,3];
        var c=[5,6,7];
        var b=arr.concat(c); //var b=arr.concat(4); 1,2,3,4
        console.log(arr);//1,2,3
        console.log(b);//1,2,3,5,6,7

1-6 删

pop();
shift();
splice(index,howmany);

                var arr=[1,2,3,4];
        arr.pop();//1,2,3
        arr.shift();//2,3,4
        console.log(arr);

                 // splice(index howmany)
                //定点删除
        var arr=[1,2,3,4,5];
        arr.splice(3,1);
        console.log(arr);//1,2,3,5

1-8 改

splice(index,howmany,...value)

                var arr=[1,2,3,4,5];
        // arr[2]=7;  1,2,7,4,5
        //splice (index,howmany,...value)
        arr.splice(2,1,7,8);
        console.log(arr);//1,2,7,8,4,5

1-9 查(下标 <==>元素)

indexOf();
slice(startIndex,endIndex)
includes() //判断数组是否包含某个值

                var arr=[1,2,3,4];
        //slice(startIndex,endIndex)不包括最后一个
        console.log(arr.indexOf(4));//3
        console.log(arr.slice(1,3));//2,3
        console.log(arr.slice(1));//2,3,4

1-10 join

                var p=document.getElementById("p");
        var arr=[1994,"html","css","js"];
        var b=arr.join();
        console.log(b);//   1994,html,css,js
        var str=arr.join("/");
        p.innerHTML=str;
        console.log(str);    //  1994/html/css/js
 例子
            var data={
            country:"美国",
            year:"1994",
            story:"犯罪"
        }
        var arr=[]
        for(var i in data){
            arr.push(data[i])
        }
        var str=arr.join("/");
        console.log(str);  //美国/1994/犯罪

1-11 升序降序

                //升序
        // arr.sort((a,b)=>{
        //     return a-b;
        // })
        var arr=[4,1,6,12,7,9];
        var res=arr.sort((a,b)=>{
            return b-a;
        })
        console.log(res); //[12, 9, 7, 6, 4, 1]
  例子(年龄排序)
           var students=[
            {name:"zhang",age:19},
                {name:"meng",age:21},
            {name:"li",age:8},
            {name:"liu",age:14},
        ]
        var result=students.sort((a,b)=>{
            return a["age"]-b["age"]
        })
        console.log(result)
  例子(外卖)
                    <button id="like">like</button>
    <button id="price">price </button>
    <button id="distance">distance</button>
    <script>
        var like=document.getElementById("like");
        var sales=[
            {name:"zs",like:56,distance:90,price:75},
            {name:"ls",like:89,distance:56,price:45},
            {name:"ww",like:76,distance:50,price:65},
        ]
        function addOrder(arr,value){
            if(Array.isArray(arr)){
                var res =arr.sort((a,b)=>{
                return a[value]-b[value];
            })
            return res;
            }else{
                return null;
            }
        }
        like.onclick=function(){
            console.log(addOrder(sales,"like"))
        }
        price.onclick=function(){
            console.log(addOrder(sales,"price"))
        }
        distance.onclick=function(){
            console.log(addOrder(sales,"distance"))
        }

1-12 reduce 求和

            1.    var arr=[1,2,3,4];
        var sum =arr.reduce((a,b)=>{
            return a+b;
        })
        console.log(sum);//10
        console.log(arr); //1,2,3,4
      2. var arr=[1,2,3,4];
        var sum=0;
        for(var i=0;i<arr.length;i++){
            sum=sum+arr[i];
        }
        console.log(sum);

1-13 取最大值

                var arr=[1,2,3];
        console.log(Math.max(45,2,10));//45
        console.log(Math.max(...arr));//3

1-14 数组的遍历

                var arr=[1,2,3,4];
        arr.forEach((item,index)=>{
            console.log(item+"--"+index)
            item+=2;
        })//1--0 2--1 3--2 4--3

1-15 of

                var arr=[1,2,3];
        for(var i in arr){
            // console.log(i)
        }
        var res=arr.map(item=>{
            return item*2
        })
        console.log(arr);
        console.log(res);2 4 6        
例子
方法1
                 var arr=[
            {name:"html",value:"12px"},
            {name:"css",value:"13px"},
            {name:"vue",value:"11px"},
        ]
        // var arr=[
        //     {name:"html",value:24},
        //     {name:"css",value:26},
        //     {name:"vue",value:22},
        // ]
        arr.map(item=>{
            var value=parseInt(item["value"])*2;
            return item["value"]=value;
        })
        console.log(arr);
方法2
var arr = [
            { name: "html", value: "12px" },
            { name: "css", value: "13px" },
            { name: "vue", value: "11px" },
        ]
        var res=[]
        for(var key in arr){
            arr[key].value=parseInt(arr[key].value)*2;
            res.push(arr[key]);
        }
        console.log(res);

1-16 map

                 var arr=[1,2,3];
        var res=arr.map(item=>{
            return item*3;
        })
        console.log(arr);
        console.log(res);
        var obj=[{name:"meng",age:21},{age:"zhang",age:13}];
        var result=obj.map(item=>{
            return item.age=0;
        })
        console.log(result);
        console.log(obj);