create dynamic multidimmensional associative array in javascript from json

Question

I get a JSON object from PHP that holds multiple rows like this:

id | type   | amount| age
1  |  abc   |   1    |  1
2  |  def   |   2    |  1
3  |  def   |   4    |  2
4  |  def   |   13   |  3

I run it through JavaScript:

$.ajax({
    type: 'POST',
    url: "test.php",
    dataType: 'json',
    success: function (data) {
        var arr = [];
        for (var i = 0; i < data.length; i++) {
            arr[data[i].type][data[i].age] = data[i].ammount;
        }
    }
});

The idea is to get array that looks like this:

arr[def][1] = 2
arr[def][2] = 4
arr[def][3] = 13
arr[abc][1] = 1
arr[abc][2] = 0 // note that this one is just empty because it was not declared from json data.

Unfortunately I get an error:

Uncaught TypeError: Cannot set property '1' of undefined

1 is [data[i].age] and if I change its value error appears with other number.

Answer 1

You should do:

var arr = {};    
for (var i = 0; i < data.length; i++) {
   var type = data[i].type,
       age = data[i].age,
       amount = data[i].ammount;
   if(!arr[type]) arr[type] = {};  //Create arr[type] if it doesn't exist
   arr[type][age] = amount;
}

You could fill the empty cells with 0 , but I'd better advise to do this for reading:

//returns the value or 0
function readArr(arr, type, age){
   if(!arr[type]) return 0;
   return arr[type][age] || 0;  
}

Then, instead of reading with arr["abc"][2] , you'd read with readArr(arr, "abc", 2) .

(If you prefer, of course you can avoid declaring a function but use its logic inline, you'd do something like (arr["abc"] ? arr["abc"][2] || 0 : 0) )

Cheers, from La Paz, Bolivia

Answer 2

I think that's because arr['abc'] (or whatever the key is) is still undefined to be an array (or JSON object) when you set its abc property or key.

var arr = [];
for (var i = 0; i < data.length; i++) {
  if (!arr[data[i].type]) {
    arr[data[i].type] = [];
  }
  arr[data[i].type][data[i].age] = data[i].ammount;
}

Or if you want to use JSON objects

var arr = {};
for (var i = 0; i < data.length; i++) {
  if (!arr[data[i].type]) {
    arr[data[i].type] = {};
  }
  arr[data[i].type][data[i].age] = data[i].ammount;
}