Automatically pass all same-domain links through unpoly.js

boost.unpoly.js

  // Automatically bind internal links to `up-follow` behaviour
  // Unpoly will already catch and handle links with explicit `up-follow` etc,
  // so we are safe to assume that if our handler below is triggered, the link
  // doesn't already have an up- attribute.
  //
  // Scenarios to handle:
  // Hat-tip to https://github.com/instantpage/instant.page/blob/master/instantpage.js
  // ✅ cmd+click
  // ✅ empty links
  // ✅ links to another domain
  // ✅ links to non-http|s protocol
  // ✅ links to http from https
  // ✅ links with anchors to existing page
  // ✅ links with `download` attribute
  // ✅ links with `target` attribute that is not equal to `_self`
  // ✅ links to non-html (in our case, we can just assume ANY extension should be skipped)
  // ✅ specifically disabled via `data-up="false"`
  //
  // Roadmap / potential to-dos:
  // - blacklist certain urls/patterns
  // - whitelist extensions
  // - enable up-instant
  up.on('click', 'a', function(event, element) {

    up.log.debug('Attempting to automatically up-follow');

    const location = window.location;

    if(event.which > 1 || event.metaKey || event.ctrlKey) {
      up.log.debug('Skipping: cmd+click');
      return;
    }

    if( ! element.href) {
      up.log.debug('Skipping: non-existent href attribute');
      return;
    }

    if(element.origin != location.origin) {
      up.log.debug('Skipping: external domain');
      return;
    }

    if( ! ['http:', 'https:'].includes(element.protocol)) {
      up.log.debug('Skipping: non http|s protocol');
      return;
    }

    if(element.protocol == 'http:' && location.protocol == 'https:') {
      up.log.debug('Skipping: from https to http');
      return;
    }

    if(element.hash && element.pathname + element.search == location.pathname + location.search) {
      up.log.debug('Skipping: anchor link to same page');
      return;
    }

    if(element.getAttribute('download') !== null) {
      up.log.debug('Skipping: download attribute is present');
      return;
    }

    if(element.target && element.target.toLowerCase() !== '_self') {
      up.log.debug('Skipping: target attribute is present and not `_self`');
      return;
    }

    if(element.pathname.split('/').pop().indexOf('.') > -1) {
      up.log.debug('Skipping: file extension detected');
      return;
    }

    if('up' in element.dataset && element.dataset.up == 'false') {
      up.log.debug('Skipping: disabled via `data-up="false"`');
      return;
    }

    up.log.debug('Automatically up-following');
    event.preventDefault();
    up.follow(element);
  });