Defers execution of a callback until a condition is true

defer-until.js

var defer_until = function(conditional,
                          callback,
                          timeout) {
  timeout = timeout || 10; // Default the timeout to 10ms 

  // Build a closure for this defer job
  var timeout_callback = function() {
    // If the conditional is true, fire off the callback
    if(conditional()) {
      callback();
    // otherwise, create a new timeout
    } else {
      setTimeout(timeout_callback, timeout);
    }
  }

  // Fire off the fire check of the conditional
  timeout_callback();
}

glenns_solution.js

var x, y;

jQuery.getJSON(url1, function(data) { x = data; render_view(x, y); });
jQuery.getJSON(url2, function(data) { y = data; render_view(x, y); });

function render_view(x, y) {
   if(x && y) {
       // Do rendering
   }
}

usage.js

var x, y;

jQuery.getJSON(url1, function(data) { x = data; });
jQuery.getJSON(url2, function(data) { y = data; });

defer_until(function() { return x && y; }, function() { render_view(x, y); }, 100);