Code inside DOMContentLoaded event not working

问题

I have used

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8" />
</head>
<body>
  <button type="button" id="button">Click</button>
  <pre id="output">Not Loading...</pre>

  <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.17.0/babel.min.js"></script>
  <script type="text/babel">
    document.addEventListener('DOMContentLoaded', function () {
      const button = document.getElementById('button');
      const output = document.getElementById('output');

      output.textContent = 'Loading...';

      addEventListener('click', function () {
        output.textContent = 'Done';
      });
     });
  </script>
</body>
</html>

but it seems the code inside document.addEventListener('DOMContentLoaded', function () {}); is not loading.

If I remove this from my code, it suddenly works.

I have made a JS Bin here.

回答1:

It's most likely because the DOMContentLoaded event was already fired at this point. Best practice in general is to check for document.readyState to determine whether or not you need to listen for that event at all.

if( document.readyState !== 'loading' ) {
    console.log( 'document is already ready, just execute code here' );
    myInitCode();
} else {
    document.addEventListener('DOMContentLoaded', function () {
        console.log( 'document was not ready, place code here' );
        myInitCode();
    });
}

function myInitCode() {}

回答2:

The event has already fired by the time that code hooks it. The way Babel standalone works is by responding to DOMContentLoaded by finding and executing all of the type="text/babel" scripts on the page. You can see this in the index.js file:

// Listen for load event if we're in a browser and then kick off finding and
// running of scripts with "text/babel" type.
const transformScriptTags = () => runScripts(transform);
if (typeof window !== 'undefined' && window && window.addEventListener) {
  window.addEventListener('DOMContentLoaded', transformScriptTags, false);
}

Just run the code directly, without waiting for the event, since you know Babel standalone will wait for it for you.

Also note that if you put you script at the end of the body, just before the closing </body> tag, there's no need to wait for DOMContentLoaded even if you don't use Babel. All of the elements defined above the script will exist and be available to your script.

In a comment you've asked:

But I am using Babel standalone in development, but I will pre-compile it when I go into production. Should I add it back on when I go into production?

Just ensure that your script tag is at the end of body as described above, and there's no need to use the event.

If it's important to you to use it anyway, you can check to see whether the event has already run by checking document.readyState (after following the link, scroll up a bit):

function onReady() {
    // ...your code here...
}
if (document.readyState !== "loading") {
    onReady(); // Or setTimeout(onReady, 0); if you want it consistently async
} else {
    document.addEventListener("DOMContentLoaded", onReady);
}

document.readyState goes through these stages (scroll up slightly from the link above):

Returns "loading" while the Document is loading, "interactive" once it is finished parsing but still loading sub-resources, and "complete" once it has loaded.

回答3:

Thanks to Ruslan & here is the full code snippet with the convenient detach of the DOMContentLoaded handler after it is used.

'use strict';
var dclhandler = false;
if (document.readyState !== 'loading') {
    start();
} else {
    dclhandler = true;
    document.addEventListener('DOMContentLoaded', start);
}
function start() {
    if (dclhandler) { document.removeEventListener('DOMContentLoaded', start); }
    console.log('Start the site`s JS activities');
}

来源：https://stackoverflow.com/questions/39993676/code-inside-domcontentloaded-event-not-working

标签

javascript

babeljs

addeventlistener

domcontentloaded