feat: typescript 6.0 support by jasonlyu123 · Pull Request #2985 · sveltejs/language-tools

jasonlyu123 · 2026-03-24T03:59:20Z

User Migration Guide

TypeScript is now a peerDependency

For users manually installing the language server, this includes VSCode users using the "svelte.language-server.ls-path" config. You also need to install TypeScript for the language server if it's not automatically installed.

TypeScript 6

See https://devblogs.microsoft.com/typescript/announcing-typescript-6-0/#breaking-changes-and-deprecations-in-typescript-6.0

Most notably:

types now defaults to []
strict is on by default. If you want to disable it. Add strict: false to your tsconfig.json/jsconfig.json.

PR details

This changes our moduleResolution config to node16 or bundler. It's mostly just some config changes. Only problem is estree-walker. The types for estree-walker v2 can't be resolved under node16, but v3 is ESM only, so copying the type for now.

As for bundling TypeScript 6, this is definitely a breaking change for the extension. Is there any other breaking change we want to ship with this? ~~And what about language-server? Is this considered a breaking change if both are allowed? Actually don't know if || works in dependencies.~~ This will also move typescript to peerDependency, so it'll be a breaking change.

…nto ts-6.0

changeset-bot · 2026-03-24T03:59:24Z

🦋 Changeset detected

Latest commit: 0f1ce0b

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

This PR includes changesets to release 4 packages

Name	Type
svelte-language-server	Minor
svelte-check	Patch
svelte2tsx	Patch
typescript-svelte-plugin	Patch

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teemingc · 2026-03-24T12:39:41Z

        "svelte": "^4.2.19",
        "svelte2tsx": "workspace:~",
-        "typescript": "^5.9.2",
+        "typescript": "^5.9.2 || ^6.0.2",


I'm not sure if the || syntax works here. Is the intention to provide support for both? It might be better to specify this as a peerDep instead 🤔 though I'm not sure how that works with VS Code extensions

Yeah, it's meant to support both. I tested by installing a git repository, and it seems to be working as I intended: By default, it installs 6.0 and can be manually overridden to be 5.9. I think we can also move it to peerDep and mark it a breaking change?

peer dep sounds good, though we gotta do language-server package as a minor then

jasonlyu123 · 2026-03-25T06:55:19Z

About the failed test. I narrowed down the problem to svelteHTML.mapElementTag. For some reason, TypeScript try to create a union for every property of HTMLElement to its derived types. For example, HTMLElement.hidden and HTMLDivElement.hidden. This adds around 10ms (about 30%) on my machine for each iteration of the performance test. But it only runs once during the type-checker's lifetime, i.e. every keystroke. 10ms every keystroke is not small, but it's not really that noticeable. I'll still try to create an issue in TypeScript, but I don't think this is a major blocker.

Edit: Just noticed TypeScript team has updated the README with a merge criteria for the old codebase. Guess we have missed the beta/RC phase to get this fixed 😅.

…nto ts-6.0

jasonlyu123 · 2026-03-26T08:01:00Z

Draft until the next release. Wanted to release the current patches before we update the extension to use TypeScript 6.

it might have run too fast

…nto ts-6.0

closes #15372 This PR extends the SvelteKit peerDep range to include TS 6.0. It also upgrades our apps/packages using TS purely as a devDependency to use 6.0 (this excludes `kit` because we use TS to generate public types and that can be a breaking change. `package` is also excluded because it can't support TS6 until svelte2tsx does). ### TODOs - [x] update `svelte-check` sveltejs/language-tools#2988 Required for `@sveltejs/package` if we want to support TS6 in the future: - [ ] update `svelte2tsx` sveltejs/language-tools#2985 - [ ] update `svelte-preprocess` sveltejs/svelte-preprocess#675 Required for SvelteKit 3 when we switch over to TS6 completely - [x] update `@sveltejs/eslint-config` sveltejs/eslint-config#67 - [ ] update `dts-buddy` sveltejs/dts-buddy#123 --- ### Please don't delete this checklist! 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 - [ ] This message body should clearly illustrate what problems it solves. - [ ] Ideally, include a test that fails without this PR but passes with it. ### Tests - [ ] Run the tests with `pnpm test` and lint the project with `pnpm lint` and `pnpm check` ### Changesets - [ ] If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running `pnpm changeset` and following the prompts. Changesets that add features should be `minor` and those that fix bugs should be `patch`. Please prefix changeset messages with `feat:`, `fix:`, or `chore:`. ### Edits - [ ] Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

dummdidumm · 2026-04-02T14:12:50Z

@@ -1,5 +1,6 @@
 {
    "compilerOptions": {
+        "strict": false,


one day we will mark this strict and fight all the errors that come from it 😆

dummdidumm · 2026-04-02T14:14:20Z

I narrowed down the problem to svelteHTML.mapElementTag.

Mhhhm can we adjust the type in a way that it's not that costly anymore? Feels kinda bad that it will regress this much per keystroke.

jasonlyu123 · 2026-04-02T14:39:55Z

Mhhhm can we adjust the type in a way that it's not that costly anymore

Maybe we could still try our luck to open an issue. Not entirely sure why it does that. document.createElement doesn't, on the other hand. And it also doesn't appear in typescript-go, although the hot path has a corresponding code in typescript-go. I tried debugging with the debugger, but the same svelteHTML.mapElementTag doesn't trigger the problem during type-check.

jasonlyu123 · 2026-04-03T15:34:31Z

Pretty weird that the performance problem also happened in typescript go on another PC. I created
microsoft/TypeScript#63347. Hope at least it can be improved in typescript go.

jasonlyu123 · 2026-04-04T04:05:25Z

Hmm. It actually isn't caused by runtime code changes in TypeScript 6, but the DOM definition update. Might be an existing problem that gets triggered by the DOM update.

TypeScript 6 is also noticeably faster in some areas. Besides the performance test, our language server test is reliably faster with TypeScript 6. I also just noticed that the benchmark in the performance test is faster, so part of the reason it failed is that the target time has also decreased.

…nto ts-6.0

shouldn't matter in most cases but still restore the original logic

…nto ts-6.0

jasonlyu123 · 2026-05-01T14:08:54Z

I'll merge this next week if there's no objection. The upstream issue for the mapElement problem mentioned is closed as "Not a defect", so it's probably not possible to fix the root cause of the problem in TypeScript 6.

… branch 'master' of https://github.com/sveltejs/language-tools into ts-6.0

jasonlyu123 added 6 commits February 13, 2026 12:25

wip

e433559

Merge branch 'master' of https://github.com/sveltejs/language-tools i…

026cd33

…nto ts-6.0

bumps

7e093ce

another default change

83f3ec2

changeset

9bc6cff

format

2c578bb

jasonlyu123 added 2 commits March 24, 2026 12:41

fix svelte2tsx build for now

120ca56

more svelte2tsx migrate

41bef87

teemingc reviewed Mar 24, 2026

View reviewed changes

Comment thread packages/language-server/tsconfig.json Outdated

teemingc reviewed Mar 24, 2026

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Comment thread packages/svelte2tsx/test/tsconfig.json

add node to be explicit

c9b0987

teemingc mentioned this pull request Mar 24, 2026

feat: TypeScript 6.0 sveltejs/kit#15595

Merged

11 tasks

jasonlyu123 mentioned this pull request Mar 24, 2026

fix: typescript 6.0 compatibility #2988

Merged

teemingc reviewed Mar 24, 2026

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jasonlyu123 added 4 commits March 25, 2026 15:08

avoid svelteHTML.mapElementTag in perf test for now

02eadea

update svelte5 snapshot

4209e42

update svelte-check smoke test to bundler

e689a42

Merge branch 'master' of https://github.com/sveltejs/language-tools i…

c1fed4f

…nto ts-6.0

jasonlyu123 mentioned this pull request Mar 26, 2026

Allow subpath imports that start with #/ #2991

Closed

ocavue mentioned this pull request Mar 26, 2026

chore: update typescript to v6 prosekit/prosekit#1446

Merged

jasonlyu123 added 2 commits March 27, 2026 23:20

try fix flaky test

9199b47

it might have run too fast

Merge branch 'master' of https://github.com/sveltejs/language-tools i…

36742a6

…nto ts-6.0

jasonlyu123 marked this pull request as ready for review March 31, 2026 02:14

dummdidumm reviewed Apr 2, 2026

View reviewed changes

move typescript to peer in language server

b224592

jasonlyu123 added 2 commits April 10, 2026 21:56

Merge branch 'master' of https://github.com/sveltejs/language-tools i…

24d9421

…nto ts-6.0

another drive-by fix

c003ae9

shouldn't matter in most cases but still restore the original logic

jasonlyu123 mentioned this pull request Apr 14, 2026

support typescript 6 #3006

Closed

GrygrFlzr mentioned this pull request Apr 17, 2026

Create release for @sveltejs/package sveltejs/kit#15720

Closed

ocavue mentioned this pull request Apr 23, 2026

Widen typescript peer dependency range to allow v6 withastro/astro#16471

Merged

simonrey1 mentioned this pull request Apr 29, 2026

fix(deps): Dependabot does not update TypeScript podman-desktop/podman-desktop#17248

Open

jasonlyu123 added 2 commits April 30, 2026 16:37

Merge branch 'master' of https://github.com/sveltejs/language-tools i…

7a659b1

…nto ts-6.0

use 6.0.3

7d232bc

james-pre mentioned this pull request May 4, 2026

Update to Temporal james-pre/axium#10

Open

Merge branch 'ts-6.0' of https://github.com/jasonlyu123/language-tools;…

0f1ce0b

… branch 'master' of https://github.com/sveltejs/language-tools into ts-6.0

jasonlyu123 merged commit 29e2894 into sveltejs:master May 5, 2026
3 checks passed

jasonlyu123 deleted the ts-6.0 branch May 5, 2026 01:00

github-actions Bot mentioned this pull request May 5, 2026

Version Packages #3015

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User Migration Guide

TypeScript is now a peerDependency

TypeScript 6

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