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No prefetching occurs for duplicate links #387

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Pimm opened this issue May 9, 2023 · 0 comments · May be fixed by #403
Open

No prefetching occurs for duplicate links #387

Pimm opened this issue May 9, 2023 · 0 comments · May be fixed by #403

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@Pimm
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Pimm commented May 9, 2023 •
edited
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Describe the bug

When a link appears on the page twice ‒ once inside and once outside the viewport ‒ and a delay has been set, the link will not cause prefetching/prerendering.

To Reproduce

<!doctype html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <link rel="stylesheet" href="main.css">
</head>

<body>
  <div class="screen">
    <a href="1.html">Link</a>
  </div>
  <a href="1.html">Same link</a>
  <script src="../../dist/quicklink.umd.js"></script>
  <script>
    quicklink.listen({ delay: 1000 });
  </script>
</body>

</html>

Expected behavior

The page to be prefetched, as at least one link to it is in-viewport.

The IntersectionObserver detects that the first link is in-viewport, and appropriately pushes into the hrefsInViewport array. It immediately afterwards detects that the second link is not in-viewport, and thus removes the freshly added item from hrefsInViewport. hrefsInViewport ‒ as the name suggests ‒ contains hrefs, thus links with equal hrefs are indistinguishable in this context.

I am willing to submit a PR, but would like to make I'm not the only one who thinks this is unexpected.

One possible solution would be storing some identifier for a link that is unique to it, rather than the href (which is not necessarily unique). Such an identifier could be generated like so:

const identify = function(ids, node) {
  let id = ids.get(node);
  if (!id) {
    ids.set(node, id = {});
  }
  return id;
}.bind(undefined, new WeakMap());
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Fix duplicate links not being preloaded daun/quicklink
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@Pimm