Run code after a dependancy has been injected into the page

depend.js

function depend( url , callback )
{
  var script = document.createElement( 'script' );
  var scripts = document.getElementsByTagName( 'script' )[0];
  script.async = true;
  script.onload = function()
  {
    script.onload = null;
    callback();
  }
  script.src = url;
  ( document.getElementsByTagName( "head" )[ 0 ] ).appendChild( script );
}