What you’re looking for is called a post decorator. It’s a hook that allows you to run scripts before each post is rendered. It’s a part of the plugin API.
Now, regarding what you want to do (redirect all external links). I don’t think adding that much friction is a good idea, so I can’t help you with that - plus you already figured that bit out. That said, here’s a commented example of how you can create a setup that targets all external links in posts
This goes in the header section of your theme / theme component
<script type="text/discourse-plugin" version="0.8">
// store the hostname so we can reuse it.
const siteHostname = location.hostname;
// let's create a decorator to do this in every post
api.decorateCooked(
post => {
// does the post have links?
const links = [...post[0].querySelectorAll("a")];
// no links, bail.
if (!links.length) return;
// We have links, let's filter them and only grab the ones that are external
const externalLinks = links.filter(
link => link.hostname !== siteHostname
);
// if we have some external links, let's do some work. For example, we can
// add a class to each external link like so.
externalLinks.forEach(link => {
link.classList.add('external-link');
// do more work here.
});
},
// we give our decorator an id to prevent memory leaks.
{ id: "external-link-decorator" }
);
</script>
Again, redirecting users is too much friction and will get annoying real fast. So, I would consider another approach, like maybe adding a subtle icon next to external links and teaching your users what it means instead?
