widget.js

(function( global ) {
  var scriptRootNode;
  var scriptNamespace = 'embed';
  var scriptNameRegex = /.*embed.js/;
  var scriptPath = 'https://domain.tld/widget-app.js';
  var stylePath = 'https://domain.tld/widget-app.css';

  if ( ! global[ scriptNamespace ] )
    global[ scriptNamespace ] = {};

  var scriptEmbed = global[ scriptNamespace ];

  if ( ! scriptEmbed.isEmbedded )
    scriptEmbed.isEmbedded = false;

  var scriptTags = document.getElementsByTagName( 'script' );

  var loadJavaScriptResource = function( resourcePath, onLoadCallback ) {
    var scriptNode = document.createElement( 'script' );
    scriptNode.type = 'text/javascript';
    scriptNode.src = resourcePath;
    scriptNode.charset = 'utf-8';
    scriptNode.async = false;

    if ( typeof onLoadCallback === 'function' )
      scriptNode.onload = onLoadCallback;

    document.getElementsByTagName( 'head' )[ 0 ].appendChild( scriptNode );
  };

  var loadStylesheetResource = function( resourcePath ) {
    var styleNode = document.createElement( 'link' );
    styleNode.type = 'text/css';
    styleNode.rel = 'stylesheet';
    styleNode.href = resourcePath;

    document.getElementsByTagName( 'head' )[ 0 ].appendChild( styleNode );
  };

  if ( scriptTags.length > 0 ) {
    for ( var i = 0, l = scriptTags.length; i < l; i++ ) {
      scriptRootNode = scriptTags[ i ];
      var matches = scriptTag.src.match( scriptNameRegex );

      if ( matches !== null && matches.length > 0 && scriptEmbed.isEmbedded === false ) {
        loadJavaScriptResource( scriptPath );
        loadStylesheetResource( stylePath );

        scriptEmbed.isEmbedded = true;
        break;
      }
    }
  }

  global[ scriptNamespace ] = scriptEmbed;
}( this ));