关于 JS 数组，对象 length 使用的坑

场景: laravel中前端请求后端返回一个数组，返回格式如下:

 $data = [ "datalist" =>
        [   16 => ["name" => "钢筋工",
              "count" => 55,
              "num" => 16 ],
            46 => [ "name" => "木工二班",
              "count" => 57,
              "num" => 46 ],
            25 =>[ "name" => "塔吊",
              "count" => 6,
              "num" => 25 ],
            31 => [ "name" => "机电",
              "count" => 5,
              "num" => 31 ],
        ],
        "other" => 0,
        "count" => 188 ,
    ];

JS中拿到数据是个对象，前端获取对象长度使用了(假如返回值用res接收)

var len = res.datalist.length

结果len的值是undefined.

解决:
方案1:

$data = [ "datalist" =>
        [   0 => ["name" => "钢筋工",
              "count" => 55,
              "num" => 16 ],
            1 => [ "name" => "木工二班",
              "count" => 57,
              "num" => 46 ],
            2 =>[ "name" => "塔吊",
              "count" => 6,
              "num" => 25 ],
            3 => [ "name" => "机电",
              "count" => 5,
              "num" => 31 ],
        ],
        "other" => 0,
        "count" => 188 ,
    ];

将datalist变成从0开始的索引数组就好了(用了函数array_values()从新排序了)，这样计算出来的长度len = 4；
方案2：

var arr = Object.keys(res.datalist);
var len = arr.length; //4

这样的结果len也是4;

总结: 因为方便写后续的代码，我选择了方案1 ，感觉JS的数组（从0按顺序排序）和对象（key值也是0,1,2排序）就是一样一样的。

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