Hook into an svg's dom. Note that you need to run this after the object tag has loaded (so you can trigger it with an onload function). It may require adaptation for non-svg elements.

exposeSVGDOM.js

/*
https://stackoverflow.com/a/50059015/8652537
*/
function hooksvg(elementID) { //Hook in the contentDocument of the svg so we can fire its internal scripts
   var svgdoc, svgwin, returnvalue = false;
   var object = (typeof elementID === 'string' ? document.getElementById(elementID) : elementID);
   if (object && object.contentDocument) {
      svgdoc = object.contentDocument;
   }
   else {
      if (typeof object.getSVGDocument == _f) {
         try {
            svgdoc = object.getSVGDocument();
         } catch (exception) {
            //console.log('Neither the HTMLObjectElement nor the GetSVGDocument interface are implemented');
         }
      }
   }
   if (svgdoc && svgdoc.defaultView) {
      svgwin = svgdoc.defaultView;
   }
   else if (object.window) {
      svgwin = object.window;
   }
   else {
      if (typeof object.getWindow == _f) {
         try {
            svgwin = object.getWindow();//TODO look at fixing this 
         }
         catch (exception) {
            // console.log('The DocumentView interface is not supported\r\n Non-W3C methods of obtaining "window" also failed');
         }
      }
   }
   //console.log('svgdoc is ' + svgdoc + ' and svgwin is ' + svgwin);
   if (typeof svgwin === _u || typeof svgwin === null) {
      returnvalue = null;
   } else {
      returnvalue = svgwin;
   }
   return returnvalue;
};

/*
If you wanted to grab the symbol elements from the dom for the svg, your onload function could look like this:

function loadedsvg(){
   var svg = hooksvg('mysvgid');
   var symbols = svg.document.getElementsByTagName('symbol');
}
*/