使用唯一键创建关联数组

I'm creating an associative array (technically an object) like this, and the keys should be strings.

var j = <?php echo $max_key ?>;
var id = "ele"+j;
eleDetailsTop[id] = {id: id, size : "100%", sizeLabel : 12};

Finally this object will be stored in the DB as a JSON string.

My problem is generating a value for j. Keys will look like this ele0, ele1, ele2...etc. So, value of j It should be +1 from the max value of the currently stored keys in the DB. Let's say the max value is ele4, the next value of j should be 5. So, then I can generate new id as ele5

How to get this done ?

var max = <?php echo $max_key ?>;
var eleDetailsTop = [];
for (var id = 0; id < max; id++) {
  eleDetailsTop["ele" + id] = {id: id, size : "100%", sizeLabel : 12};
}