title: Getting html from cross domain iframe window
body: <p>I have an iframe in my index file and I want fetch all of the html from the iframe and store it in a variable. The content loaded inside of the iframe is from another domain. So I am trying to achieve this using javascript and postMessage and an eventListener. This is my code so far. I have a button which calls the click method. </p>
  
  <p>index.vue</p>
  
  <pre><code>    click() {
          let iframe = document.getElementById('the_iframe');
          iframe.contentWindow.postMessage('Dont really know what to send here', '*');
          this.test();
      },
      test() {
          window.addEventListener('message', function(event) {
              //Just a test, as the it wont go in the else
              console.log(event.currentTarget.document);
              if (event.origin !== 'http://testdomain') {
                  console.log('ERROR');
              } else {
                  console.log(event.currentTarget.document);
                  const html = event.currentTarget.document); 
                  //This however doesnt work. The value is not stored in the const
                  console.log(event.currentTarget.document);
              }
          });
      }
  </code></pre>
  
  <p>When I console log event.currentTarget.document it prints out the entire html structure of the site, including the parent page (index). My question is: How do i fetch all of the html of just the iframe inside of my index file? and how do I store it in a variable? </p>