Populating array with response from nested AJAX calls

So, dict is an object that has no push method. You'd need dict=[]; If you had to have {}, then you'd need key:value pairs to populate it, such as:

dict[key] = value;

You are going to have to keep track of the number of calls that you are doing in that for loop and fire a callback function once they are all complete. I'm not totally confident about your current solution, with calling an indefinite amount of ajax requests, but I also don't fully understand the scope of your problem or the server that you're talking to.

So basically you will have to do something like this with what you have currently:

var dict = [],
  requestsCompleted = 0;


function dictFilled() {
  // do something with your dict variable;
}

function fillArray(){
  $.post('getUsersOnline.php', function(phpReturnVal){
    // ...
    for(var i = 0; i < phpReturnVal.length; i++){
        $.get("https://api.twitch.tv/kraken/streams"  , function(data){
          dict[data.key] = data;
          requestsCompleted++;
          if (requestsCompleted === phpReturnVal.length) {
            dictFilled();
          }
    });
  });
}

This haven't been tested, but basically you will have to define a function that will have access to the array that you are filling and call it once all you asynchronous requests finish successfully. For tasks like this though I recommend you take a look at jQuery's Deferred API. There is always a chance that one of those requests will fail and your application should know what to do if that happens.

I'm assuming the GET call is asynchronous and doesn't happen until after all the JS code has executed...

Appear correct.

Try

var dict = [];

function fillArray() {
  // return jQuery promise object
  return $.post('getUsersOnline.php', function(phpReturnVal){
    // ...
    for(var i = 0; i < phpReturnVal.length; i++) {
        // call same `url` `phpReturnVal.length` times here ?,
        // returning same `data` response at each request ?,
        // populating, overwriting `dict` with same `data.key` property ?
        $.get("https://api.twitch.tv/kraken/streams"  , function(data) {
          dict[data.key] = data;
        });
    }; // note closing `}` at `for` loop
    // return value
    return dict
  });
}

function doStuff(data) {
   // `data`:`dict`, promise value returned from `fillArray`
   console.log(data);
}

fillArray().then(doStuff, function error(jqxhr, textStatus, errorThrown) {
  console.log(textStatus, errorThrown)
});

var arr = ["a", "b", "c"];
var response = {
  "a": 1,
  "b": 2,
  "c": 3
};
var obj = {};
var dict = [];

function fillArray() {
  return $.when(arr).then(function(phpReturnVal) {
    for (var i = 0; i < phpReturnVal.length; i++) {
      // return same `response` here ?
      $.when(response).then(function(data) {
        dict[arr[i]] = data;
      });
    };
    return dict
  });
}

function doStuff(data) {
  console.log(data)
}

fillArray().then(doStuff, function error(err) {
  console.log(err)
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js">
</script>