Unwanted array indexes when using JSON.stringify

Question

I use JSON.stringify method to pass an array to server.

I have an array that has 4 elements:

arr[10] = 1;
arr[20] = 1;
arr[30] = 1;
arr[40] = 1;

Then i do this:

arr = JSON.stringify(arr);

Then send it to the server:

                jQuery.ajax({
                    type: "post",
                    url: baseurl+"profile/mprofile/action/ratings/add_ratings",
                    data:{"checkbox":checkbox,"review":review,"speciality":speciality,"arr":arr},
                    success: function(data, status) {  

                        jQuery('#header-error').html(data);
                    } 
                }); 

I am getting the array in PHP with:

    $arr = $this->ci->input->post('arr');
    $arr = json_decode($arr);
    print_r($arr) ; die ;

the result is

Array
(
    [0] => 
    [1] => 
    [2] => 
    [3] => 
    [4] => 
    [5] => 
    [6] => 
    [7] => 
    [8] => 
    [9] => 
    [10] => 1
    [11] => 
    [12] => 
    [13] => 
    [14] => 
    [15] => 
    [16] => 
    [17] => 
    [18] => 
    [19] => 
    [20] => 1
    [21] => 
    [22] => 
    [23] => 
    [24] => 
    [25] => 
    [26] => 
    [27] => 
    [28] => 
    [29] => 
    [30] => 1
    [31] => 
    [32] => 
    [33] => 
    [34] => 
    [35] => 
    [36] => 
    [37] => 
    [38] => 
    [39] => 
    [40] => 1
)

Why is this happening?

Answer 1

That's how javascript arrays work. When you set value to an index that is out of range, array is expanded, value is successfully set and all 'missing' values are set to undefined.

You are spoiled by PHP behaviour that blends the difference between arrays and dictionaries (or "associative arrays", in PHP talk).

To avoid this behaviour, just create a dictionary in javascript instead of array.

var arr = {};
arr[10] = 1;
arr[20] = 2;

Also, on PHP side you should pass true as a second parameter to json_decode.

json_decode($arr, true)

This will make it return array instead of stdclass.

Answer 2

How do you initialize arr? You probably want to use an object instead of an Array. For example:

var arr = {};
arr[10] = 1;
arr[20] = 1;
arr[30] = 1;
arr[30] = 1;
var jsonified = JSON.stringify(arr);
console.log(jsonified);