Load a handlebars template into the DOM with no additional code. Gets the template URL from script's `data-template` attribute, and JSON template data URL from `data-context` attribute.

handlebars-template-loader.js

(function(){

// get a reference to the current script (IE support)
// http://stackoverflow.com/a/22745553
const scripts = document.getElementsByTagName('script');
const currentScript = scripts[ scripts.length - 1 ];
  
const CONTEXT_URL = currentScript.dataset.context;
const TEMPLATE_URL = currentScript.dataset.template;

if (!CONTEXT_URL || !TEMPLATE_URL) {
  console.error('Missing `context` or `template` data attributes.');
}
else {
  get(CONTEXT_URL, function(data) {
    const context = JSON.parse(data);
    get(TEMPLATE_URL, function(source) {
      // http://handlebarsjs.com/#getting-started
      const template = Handlebars.compile(source);
      const html = template(context);
      const app = document.createElement('div');
      app.id = 'app';
      app.innerHTML = html;
      // insert the compiled html after the script tag
      // http://stackoverflow.com/a/4793630
      currentScript.parentNode.insertBefore(app, currentScript.nextSibling);
    });
  });
}

function get(url, callback) {
  var r = new XMLHttpRequest();
  r.open('GET', url);
  r.onreadystatechange = function() { 
    if (r.readyState !== 4 || r.status !== 200) return; 
    else callback(r.responseText);
  };
  r.send();
}
  
})();