[Martin Ferret (MartinFerret)]

Part of #2566

observe() throws a TypeError whenever none of childList, attributes or characterData ends up true, so calling observe(target) without options always throws (covered by WPT in dom/nodes/MutationObserver-sanity.html). As discussed in the issue, this makes the options parameter required, which flags that mistake at compile time without affecting code that works at runtime.

Since the KDL patch format didn't support optional on method parameters yet, this PR also adds that to handleParam in src/build/patches.ts (one line), then uses it in inputfiles/patches/dom.kdl. If you'd prefer this override in overridingTypes.jsonc instead, I'm happy to move it.

Note: this only covers calling observe(target) without options. Options objects that don't meet the minimum requirements, such as {} or { subtree: true }, are still accepted by the type and throw at runtime, so I'm not closing the issue with this PR.

[github-actions]

Thanks for the PR!

This section of the codebase is owned by Kagami Sascha Rosylight (@saschanaz) - if they write a comment saying "LGTM" then it will be merged.

[Martin Ferret (MartinFerret)]

@microsoft-github-policy-service agree

[Kagami Sascha Rosylight (saschanaz)]

Thanks! LGTM.

[github-actions]

Merging because Kagami Sascha Rosylight (@saschanaz) is a code-owner of all the changes - thanks!