JS array length is defined

scene : laravel The middle front end requests the back end to return an array , The return format is as follows :

  $data = [ "datalist" =>
        [   16 => ["name" => " A steel worker ",
              "count" => 55,
              "num" => 16 ],
            46 => [ "name" => " Carpenter class 2 ",
              "count" => 57,
              "num" => 46 ],
            25 =>[ "name" => " Tower crane ",
              "count" => 6,
              "num" => 25 ],
            31 => [ "name" => " electromechanical ",
              "count" => 5,
              "num" => 31 ],
        ],
        "other" => 0,
        "count" => 188 ,
    ];

JS The data obtained in is an object , The front end gets the object length using ( If the return value is res receive ) 

var len = res.datalist.length 

result length The value of is  undefined. 

solve :

$data = [ "datalist" =>
        [   0 => ["name" => " A steel worker ",
              "count" => 55,
              "num" => 16 ],
            1 => [ "name" => " Carpenter class 2 ",
              "count" => 57,
              "num" => 46 ],
            2 =>[ "name" => " Tower crane ",
              "count" => 6,
              "num" => 25 ],
            3 => [ "name" => " electromechanical ",
              "count" => 5,
              "num" => 31 ],
        ],
        "other" => 0,
        "count" => 188 ,
    ]; 

take datalist Become from 0 Just start with the index array ( The function is used array_values () Sorted again ), The length thus calculated len = 4; 

  source ：https://learnku.com/articles/44975