feat: default values for form elements

[dummdidumm]

WIP

closes #8220
closes #9230

This implements default values for form elements. These default values take effect when the reset method of the surrounding form is invoked.

• for input with value/checked it's defaultValue/defaultChecked
• for input with bind:group it's checked (hydration not working yet, not working for dynamic yet) no handling for bind:group, because too confusing / limiting
• for select it's selected on the option elements (hydration not working yet, not working for dynamic yet)
• defaultValue/defaultChecked on the server -> use if value undefined/falsy? -> not for now

Open questions:

• for defaultValue/defaultChecked the value of that takes precedence on the initial render in case the value/checked value is undefined/null. That also means that in case of bind:value/checked the default value will be propagated back to that variable. Ok?
  • ok
• In case that's ok, should we do the same for bind:group? It's tricky there because for a group of checkboxes, we currently require an array to be set (else you get runtime errors), and so the value isn't falsy, and the empty array could be on purpose / hard to say if the default checked should take precedence there initially
  • no handling for bind:group. That binding is kinda special in many ways and with bind:checked={get, set} we can likely provide a more robust/scalable/working across components solution anyway
• Same question for bind:value on selects with multiple, with same problem as for bind:group on checkboxes
  • for multiple the initial selection is ignored, i.e. the array takes precedence

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: d4e836b

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14289-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14289

[Rich-Harris]

looks like <select> elements aren't being reset correctly? I have to press the reset button twice for the state to update, even though the select itself updates immediately

solved