URLPattern {/:optional}? no longer works as expected.

[JTRNS]

TLDR

const pattern = new URLPattern({pathname: "/books{/:genre}?" });
console.log(pattern.pathname) // PREVIOUSLY resulted in "/books{/:genre}?"
console.log(pattern.pathname) // CURRENTLY results in "/books/:genre?"

Just wanted to give you a heads up on some additional URLPattern changes that impact the current router logic inside the pathToPattern function.

// snippet from /src/router.ts
if (part[j + 1] === "[") {
  // Disallow optional dynamic params like `foo-[[bar]]`
  if (part[j - 1] !== "/" && !!part[j - 1]) {
    throw new SyntaxError(
      `Invalid route pattern: "${path}". An optional parameter needs to be a full segment.`,
    );
  }
  groupOpen++;
  optional = true;
  pattern += "{/";
  j++;
}

Haven't exactly been able to figure out why, but I did notice that the test case for this pattern was recently removed from the deno repository.

I am assuming it simply didn't comply with the specification, but not completely sure. If you have some more insight into the why this URL pattern is no longer supported I'd love to know as I was using it too.

[marvinhagemeister]

There was a recent specification change in URLPattern, see whatwg/urlpattern#198 . It allows group values to be undefined instead of always falling back to an empty string.

Taking your pattern as an example: The genre group can now be undefined when passing http://example.com/books without a genre.

// Before
> new URLPattern({pathname: "/books{/:genre}?" }).exec("https://foo.com/books")
{
  inputs: [ "https://foo.com/books" ],
  protocol: { input: "https", groups: { "0": "https" } },
  username: { input: "", groups: { "0": "" } },
  password: { input: "", groups: { "0": "" } },
  hostname: { input: "foo.com", groups: { "0": "foo.com" } },
  port: { input: "", groups: { "0": "" } },
  pathname: { input: "/books", groups: { genre: "" } },   //  <-- falls back to empty string
  search: { input: "", groups: { "0": "" } },
  hash: { input: "", groups: { "0": "" } }
}

// After
> new URLPattern({pathname: "/books{/:genre}?" }).exec("https://foo.com/books")
{
  inputs: [ "https://foo.com/books" ],
  protocol: { input: "https", groups: { "0": "https" } },
  username: { input: "", groups: { "0": "" } },
  password: { input: "", groups: { "0": "" } },
  hostname: { input: "foo.com", groups: { "0": "foo.com" } },
  port: { input: "", groups: { "0": "" } },
  pathname: { input: "/books", groups: { genre: undefined } },   //  <-- is undefined
  search: { input: "", groups: { "0": "" } },
  hash: { input: "", groups: { "0": "" } }
}

In Fresh we always normalize it to a string for now to avoid introducing a breaking change to users.