数组拼接成字符串(join)

var arr = ["html","css","javascript"];
console.log(arr.join());//html,css,javascript
console.log(arr.join(""))//htmlcssjavascript
console.log(arr.join("|"))//html|css|javascript
console.log(arr.join(" "))//html css javascript

例子

<script>
        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
</script>

数组升序、降序(sort)

sort方法会改变原arr的值

升序

var arr = [4,7,1,3,14]; 
var res = arr.sort((a,b)=>{
  return a-b;
})
console.log(res)//[1,3,4,7,14]

降序

var arr = [4,7,1,3,14]; 
var res = arr.sort((a,b)=>{
  return b-a;
})
console.log(res)//[14,7,4,3,1]

年龄升序排列

<script>
        var students = [
            {name:"zhang",age:20},
            {name:"li",age:18},
            {name:"wang",age:9},
            {name:"chen",age:14},
        ]
        /* 
        根据年龄升序
        根据年龄降序
         */
         var result = students.sort((a,b)=>{
             return a["age"]-b["age"]
         })
         console.log(result)
</script>

美团

求和(reduce)

reduce方法不会改变原arr的值

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

取最大\小值Math.max\min(…arr)

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

遍历数组

forEach

forEach方法不会改变原arr的值

<script>
        var arr = [2,3,4,5];
        arr.forEach((item,index)=>{
            console.log(item+"--"+index)
        })
</script>

of

var arr = [2,3,4,5]
for(var i of arr){
  console.log(i);
}

# of 支持异步循环
function muti(num){
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(num*num)
    },1000)
  })
}
var arr = [2,3,4];
/* 
  forEach 是一个同步的循环
  of 支持异步循环
*/
(async()=>{
  /* arr.forEach(async(item)=>{
                var res = await muti(item)
                console.log(res)
            }) */
  for(let value of arr){
    let res = await muti(value)
    console.log(res)
  }
})()

map

可以改变数组每一个项,得到一个新的值
1.数组的每一项是基础(简单)类型,执行map不会改变数组原来的结构
2.数组的每一项是复杂类型{object},那么执行map方法会改变原来的数据结构

<script>
        var arr = [1,2,3];
        var res = arr.map(item=>{
            return item*3;
        })
        console.log(arr);// [1,2,3]
        console.log(res);// [3,6,9]
        var obj = [{name:"li",age:10},{age:"zhang",age:11}];
        var result  = obj.map(item=>{
            return item.age = 0;
        })
        console.log(result);
        console.log(obj)
    </script>

例子

<script>
        var arr = [
            {name:"html",value:"12px"},
            {name:"css",value:"13px"},
            {name:"vue",value:"11px"},
        ]
        arr.map(item=>{
            var value = parseInt(item["value"])*2;
            return item["value"] = value;
        })
        console.log(arr);
                /* [
            {name:"html",value:24},
            {name:"css",value:26},
            {name:"vue",value:22},
        ]*/
</script>

判断

some 返回boolean 只有数组中一项满足条件,就输出true
every 返回boolean 只有数组中m每一项满足某个条件,就输出true

<script>
        var arr = [1,2,3,4];
        var res = arr.some(item=>{
            return item>2;
        })
        var b = arr.every(item=>item>2);
        console.log(res);//true
        console.log(b);//false
    </script>

筛选

var arr = [1,2,3,4];
var res = arr.filter(item=>item>2);
console.log(res)//[3,4]

例子

<script>
        var arr = [
            {name:"小米",price:1999},
            {name:"红米",price:999},
            {name:"iphone",price:9999},
            {name:"华为",price:3000},
        ]
        var res = arr.filter(item=>item["price"]>1000)
        console.log(res)
</script>

反转

sort方法会改变原arr的值

<script>
        var arr = [1,2,3,4]
        var res = arr.reverse()
        console.log(res)//[4,3,2,1]    
</script>

包含

<script>
        var arr = [1,2,3,4]
        console.log(arr.includes(5))//false
</script>

对象变为数组

获取对象所有key值

Object.keys();

获取对象所有value值

Object.values();

去重

1.size 获取长度
2.add 添加数据
3.chear() 清除set中所有的数据
4.delete() 删除数据
5.forEach() 遍历
6.has 与includes类似

var arr = [1,2,3,4];
var s = new Set([...arr]);