## Summary
Adds a [Browser
support](https://github.com/MathiasWP/svelte/blob/document-pipeline-draft/documentation/docs/07-misc/05-browser-support.md)
page listing the minimum browser versions Svelte's runtime and compiler
output require. The page (headline floor + per-feature requirements) is
auto-generated by `packages/svelte/scripts/generate-browser-support.js`,
which scans the code Svelte actually ships against the
[web-features](https://github.com/web-platform-dx/web-features) Baseline
dataset and resolves results to concrete browser versions via
[baseline-browser-mapping](https://github.com/web-platform-dx/baseline-browser-mapping).
CI fails if a PR moves the floor without regenerating the page.
For 5.55.9 the floor is **Baseline 2020**: Chrome/Edge 87, Firefox 83,
Safari 14.
## How it works
1. **Headline floor.** Each runtime entry from `pkg.exports` is bundled
with rollup using production conditions, then walked with TypeScript's
compiler API + TypeChecker. The walker (`browser-support.detector.js`)
flags any web-features ID the runtime references; the highest Baseline
year among them is the floor.
2. **Per-feature requirements.** Every user-facing surface is enumerated
programmatically — every `bind:*` from `binding_properties`, every
public subpackage export discovered via `pkg.exports` + `import * as
ns`, every rune from `RUNES`, and directives (`transition:` / `animate:`
/ `use:` / `@attach` / `{@html}` / custom elements). Each gets a
fixture, compiled and bundled like user code, then scanned. A row
appears only when the fixture's floor exceeds the runtime floor.
3. **Supplemental rules.** A handful of APIs `web-features` doesn't
catalogue (`getComputedStyle(...).zoom`, `box:
'device-pixel-content-box'`) are detected with the same type-aware
walker via a small `register_extra_rules` list with inline
justifications.
4. **Ignore sets.** `SAFE_TO_IGNORE` covers dataset bugs and
feature-detected APIs with graceful fallbacks. `BEHAVIORAL_IGNORE` hides
APIs reached only via specific user code from the headline (they still
surface as conditional rows); staleness-checked, so CI fails if an entry
no longer exists in the runtime.
Resolves#18198. Provides an answer to the long-standing #558.
## Test plan
- [ ] \`pnpm --filter svelte generate:browser-support\` regenerates the
page idempotently
- [ ] CI fails when a code change bumps the floor without regenerating
the page
- [ ] Page renders correctly on svelte.dev
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>
We incorrectly restore effect context after an`$derived(await ...)` if
it occurs inside an async function, but only for those, no other `await`
expressions. This is buggy and wrong. We only want to restore context
_inside_ an async derived expression (including template expressions)`
so that `await a + b` works correctly.
It's possible that this will be a breaking change (albeit not
semver-violating, because `experimental`) for some people if they are
creating effects after an `await` (other than at the top level of a
component). This is regrettable, but it should never have worked in the
first place.
## Summary
The `script_unknown_attribute` warning message used American English
(`Unrecognized`) while every other user-facing error/warning in the
compiler uses British English (`Unrecognised`):
- `options_unrecognised` error: `Unrecognised compiler option…`
- `unknown_code` warning: `is not a recognised code`
This change updates `script_unknown_attribute` to match, then
regenerates `warnings.js` and the reference documentation from the
source message file via `node scripts/process-messages`.
## Test plan
- Spelling change is isolated to the `script_unknown_attribute` warning
message
- `warnings.js` and generated docs were updated by running `node
scripts/process-messages`
- All three occurrences (source, generated JS, generated docs) are
consistent
Ref: https://github.com/sveltejs/svelte/issues/15100
Adds the tag name to the `a11y_click_events_have_key_events` warning
given when a non-interactive element has a click handler but no keyboard
events.
This already happens in `a11y_no_static_element_interactions`, and
should probably happen in many more messages.
Closes#18022
This updates the context docs to show the getter pattern for state that
may be reassigned, such as primitive state. The new example passes a
function through context so children read the current value instead of
capturing the initial value.
This is a docs-only clarification and does not change runtime behavior.
Validation:
- `git diff --check`
- `pnpm exec prettier --check
documentation/docs/06-runtime/02-context.md`
Full `pnpm test`/`pnpm lint` were not run because this only changes
documentation.
---------
Co-authored-by: Rich Harris <hello@rich-harris.dev>
### Before submitting the PR, please make sure you do the following
- [x] 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
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
- [ ] Run the tests with `pnpm test` and lint the project with `pnpm
lint`
`$state.snapshot` already clones the value returned from `toJSON`, and
its `Snapshot<T>` type reflects that return type. The `$state.snapshot`
docs now call out that behavior explicitly, including the generated
ambient types shown by editors.
Test plan: `pnpm check`; `pnpm lint`.
Fixes#18129
Co-authored-by: sakaenyeniceri5 <sakaenyeniceri5@users.noreply.github.com>
## Summary
- Fix the missing space before `<option>` in the bind documentation.
## Related issue
- N/A
## Guideline alignment
- Read `CONTRIBUTING.md` and kept this to one focused docs-only file
change.
- No behavior, test, fixture, changeset, or generated-file changes.
## Test plan
- `git diff --check`
- Not run: docs-only spacing fix.
Co-authored-by: Codex <codex@openai.com>
Per
https://github.com/sveltejs/svelte/pull/17862#issuecomment-4049752548,
this freezes the value of a derived if it was created inside a parent
effect that is now destroyed. This prevents the sort of bug where a
derived reads `foo.bar` even though `foo` is now `undefined`.
If the derived is dirty, a warning will be printed.
This PR also gets rid of some weirdness around `derived.parent` — it can
only ever be an `Effect | null`, and there's no need for
`get_derived_parent_effect`.
Blocked on https://github.com/sveltejs/kit/pull/15533 and a follow-up
that switches remote functions to use `$effect.root` (since this
effectively undoes #17171), hence draft.
### Before submitting the PR, please make sure you do the following
- [x] 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
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`
---------
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Adds documentation for using CSS custom properties with the `style:`
directive. I think this was lost at some point during the great
transition. It's only mentioned in the [best
practices](https://svelte.dev/docs/svelte/best-practices#Using-JavaScript-variables-in-CSS)
section, so I thought it would make sense to include it.
### 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:
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- [ ] 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.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
- [ ] Run the tests with `pnpm test` and lint the project with `pnpm
lint`
---------
Co-authored-by: Rich Harris <hello@rich-harris.dev>
Co-authored-by: Rich Harris <rich.harris@vercel.com>
## Description
**While investigating the compiler analysis phase for errors or missing
lines, I located an explicit `TODO fix the message here` comment in the
source code
(`packages/svelte/src/compiler/phases/2-analyze/visitors/shared/utils.js`).**
### Triggering Code
```svelte
<script module>
import { foo } from './somewhere.js';
</script>
<script>
let foo = 'conflict'; // Triggers the duplicate module import error
</script>
```
## The Bug
When a component author creates a local `let` declaration in the
instance `<script>` block that shadows a variable imported in the
`<script module>` block, the compiler emits a
`declaration_duplicate_module_import` error.
However, the error message text was misleading. It stated:
```diff
- Cannot declare a variable with the same name as an import inside `<script module>`
+ Cannot declare a variable with the same name as an import from `<script module>`
```
This incorrectly implies that the duplicate declaration itself is
happening *inside* the module script block, rather than colliding with
an import *from* that block.
## Changes Made
| Action | Target (File / Function) | Description |
| :--- | :--- | :--- |
| **Template Fix** | `packages/svelte/messages/compile-errors/script.md`
| Changed "inside" to "from" to accurately reflect the nature of the
shadow collision. |
| **Regenerated Errors** | `packages/svelte/src/compiler/errors.js` |
Ran `pnpm generate` to apply the updated markdown template into the
dictionary list. |
| **Removed TODO** | `ensure_no_module_import_conflict` function |
Removed the original `// TODO fix the message here` notation from the
validator. |
| **Test Adjustments** | `illegal-variable-declaration/errors.json` |
Updated the validator test suite snapshots to assert against the correct
error message format. |
## Validation Results
* The error correctly throws when a user replicates this component
structure.
* Vitest suite `tests/validator/test.ts` successfully asserts the new,
clearer message.
---
### Before submitting the PR, please make sure you do the following
- [x] 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
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
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lint`
# THANK YOU
This takes advantage of https://github.com/sveltejs/svelte.dev/pull/1879
and https://github.com/sveltejs/svelte.dev/pull/1880 to replace some of
the hardcoded playground links in the docs with ones that are generated
automatically from the code. This is a better user experience, and
easier to maintain.
Not every playground link is suitable for replacement, as in many cases
we gloss over implementation details. For now I'm just starting with
some easy ones
Closes#17660
### Before submitting the PR, please make sure you do the following
- [x] 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:
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
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lint`
---------
Co-authored-by: Rich Harris <rich.harris@vercel.com>
### 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.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
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lint`
---------
Co-authored-by: Rich Harris <rich.harris@vercel.com>
With this, we can add invariants to the codebase so we can identify
problems like 'this batch already has roots scheduled', which indicate a
bug somewhere, without a) needing tests for scenarios that are
inherently hard to anticipate, or b) cluttering people's prod bundles
### Summary
Fix spelling in the v5 migration guide ("useable" -> "usable").
### 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:
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [ ] Ideally, include a test that fails without this PR but passes with
it.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`). (Not applicable; docs-only change.)
### Tests and linting
- [ ] Run the tests with `pnpm test` and lint the project with `pnpm
lint`. (Not run; docs-only change.)
Co-authored-by: rohan436 <rohan.santhoshkumar@googlemail.com>
This adds a `skill: true` to the frontmatter of the bestpractices
skill/doc. This way when we sync we can concern ourselves to only the
documents that need to be skills (we will write one for sveltekit too)
without targeting specific files in the `ai-tools` repo
Based on #17727 but with a few changes:
- smaller (fewer tokens)
- removed some bits that are really just summarising existing docs
- added an explicit 'don't use these legacy features' section
- shuffled some things around a bit
---------
Co-authored-by: paoloricciuti <ricciutipaolo@gmail.com>
Co-authored-by: Federico Varano <50919335+fvarano@users.noreply.github.com>
Co-authored-by: ComputerGuy <63362464+Ocean-OS@users.noreply.github.com>
Hi!
I run into this issue today, and found the fix in
https://github.com/sveltejs/svelte/issues/16680#issuecomment-3229435832
I thought it would be a nice addition to the documentation (it's where I
looked first)
### Before submitting the PR, please make sure you do the following
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discussed ahead of time. In many cases, features are absent for a
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solves.
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it.
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changeset (`npx changeset`).
### Tests and linting
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---------
Co-authored-by: Rich Harris <hello@rich-harris.dev>
Repository https://github.com/sveltejs/template is archived and no
longer maintained.
### 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.
- [ ] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).
### Tests and linting
- [ ] Run the tests with `pnpm test` and lint the project with `pnpm
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---------
Co-authored-by: Rich Harris <hello@rich-harris.dev>
## Motivation
noticed that the documentation for the `<svelte:document>` still refers
to actions - probably would be better for beginners if it referred to
the newer attachments instead.
### 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:
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- [x] This message body should clearly illustrate what problems it
solves.
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it.
- [ ] If this PR changes code within `packages/svelte/src`, add a
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### Tests and linting
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lint`
This makes error boundaries run on the server if a new `onerror` handler
is passed to `render`. `onerror` can either synchronously or
asynchronously return a value. It should be a sanitized
JSON.stringify-able value so that it can be passed to the client for
hydration via a comment. `mount/hydrate` also get the `onerror`
property.
If no `onerror` is passed to `render` it will just throw just like
before, hence this is backwards compatible.
This work is important for SvelteKit to allow `+error.svelte` to make
use of them and in general to make boundaries properly work during SSR
(also see https://github.com/sveltejs/kit/issues/14398).
closes#15370
### Before submitting the PR, please make sure you do the following
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discussed ahead of time. In many cases, features are absent for a
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
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changeset (`npx changeset`).
### Tests and linting
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---------
Co-authored-by: Rich Harris <rich.harris@vercel.com>
## Summary
Fixes#17721
In dev mode, detect when a keyed each block has a key function that
returns different values when called multiple times for the same item
(non-idempotent). This catches the common mistake of using array
literals like `[thing.group, thing.id]` as keys, which creates a new
array object each time and will never match by reference.
- Adds new `each_key_volatile` error with helpful message explaining the
issue
- Checks key idempotency in the each block loop during dev mode
- Provides a clear error instead of the cryptic "Cannot read properties
of undefined" that occurred previously
---------
Co-authored-by: 7nik <kifiranet@gmail.com>
Rich Harris
Simon H
Mathias Picker
Paolo Ricciuti
RonGamzu
Karl Georgsen
Mukunda Rao Katta
Wesley
Rohan Santhosh Kumar
Matia
Scott Wu
Rohit Nair P
Razin Shafayet
吴杨帆
Theodore Brown
Rohan Santhosh
Gautier Ben Aïm
Andre Hora
Theodor Steiner
Elliott Johnson
Michel Heusschen