数组字转符串拼接成逗号去掉最后的逗号

    var arr=[2,3,4,5]
        var str = "";
    for (var i = 0; i < arr.length; i++) {
      str += arr[i]+ ",";
    }
    //去掉最后一个逗号(如果不需要去掉,就不用写)
    if (str.length > 0) {
     str = str.substr(0, str.length - 1);
    }
  或者直接判断

var arr = [2, 7, 9, 7, 9, 7];  
var str1 = "";  
for (let i = 0; i < arr.length; i++) {
      if (i < arr.length - 1) {
        str1 += arr[i] + ",";
      } else {
        str1 += arr[i];
      }
    }
//输出字符串的2,7,9,7,9,7
 拼接对象    

 for (let i = 0; i < idarr.length; i++) {
          let o = {};
          o.id = idarr[i];
          o.label = namearr[i];          
          that.toData.push(o);
    }
或者

this.TableData.forEach(item => {
    const newInfo = {
    id: item.attr id,
    lable: item.attr_vals.join('')
    attrs.push(newInfo)
})
 

 

 

js forEach for区别

1、循环中断差别

具体见示例代码:

<!DOCTYPE html>
<html lang="zh">

    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>js for forEach区别 </title>
    </head>

    <body>
        <script src="https://cdn.bootcss.com/lodash.js/4.17.10/lodash.min.js"></script>
        <script type="text/javascript">
            let arr = [1, 2, 3, 4]
            for(let i = 0; i < arr.length; i++) {
                if(arr[i] == 2) {
                    continue;
                    //break;
                }
                console.log(arr[i], '  for')
            }
            for(let i in arr) {
                if(i == 2) {
                    break;
                }
                console.log(arr[i], '   for in')
            }
            for(let i of arr) {
                if(i == 2) {
                    break;
                }
                console.log(i, '   for of')
            }
            arr.forEach(val => {
                if(val == 2) {
                    //此处的return false 仅仅相当于continue
                    return false;
                    //break或者continue均不能使用 会报错,对于map filter方法一样会报错
                    //break;
                    //continue
                }
                console.log(val, '   forEach')
            })
        </script>
    </body>

</html>

数组的迭代方法:every、filter、forEach、map、some均不能使用break或者continue进行中断循环。

以上几个函数的参数都是:一个回调函数 和 一个this的指向

array.map(function(currentValue,index,arr), thisValue)

 

2、数组变化时差别

(1)数组添加操作

<!DOCTYPE html>
<html lang="zh">

    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>js for forEach区别 </title>
    </head>

    <body>
        <script src="https://cdn.bootcss.com/lodash.js/4.17.10/lodash.min.js"></script>
        <script type="text/javascript">
            let arr = [1, 2, 3, 4]
            for(let i = 0; i < arr.length; i++) {
                if(arr[i] == 2) {
                    //对于添加时,for可以遍历新数组
                    arr.push(5)
                }
                // 输出1 2 3 4 5
                console.log(arr[i], '  for')
            }

            arr.forEach(val => {
                if(val == 2) {
                    //对于添加时,forEach
                    arr.push(5)
                }
                // 输出1 2 3 4 5
                console.log(val, '   forEach')
            })
        </script>
    </body>

</html>

(2)数组更新、删除操作

<!DOCTYPE html>
<html lang="zh">

    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>js for forEach区别 </title>
    </head>

    <body>
        <script src="https://cdn.bootcss.com/lodash.js/4.17.10/lodash.min.js"></script>
        <script type="text/javascript">
            let arr = [1, 2, 3, 4]
            for(let i = 0; i < arr.length; i++) {
                if(arr[i] == 2) {
                    //对于更新、删除时,for可以遍历新数组
                    arr[1] = 100
                    //arr.splice(i,1)
                }
                // 输出1 100 3 4
                console.log(arr[i], '  for')
            }

            arr.forEach((val, i) => {
                if(val == 2) {
                    //对于更新、删除时,forEach可以遍历新数组
                    val = 100
                    //arr.splice(i,1)
                }
                // 输出1 100 3 4
                console.log(val, '   forEach')
            })
        </script>
    </body>

</html>

 

 

 

 

 

 

 

 

 

 

 

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