ref(sveltekit): Replace recast + @babel/parser with acorn by roli-lpci · Pull Request #19533 · getsentry/sentry-javascript

roli-lpci · 2026-02-26T10:41:06Z

Replaces recast and @babel/parser with acorn and @sveltejs/acorn-typescript for AST parsing in the sveltekit auto-instrumentation plugin.

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Ref #19447

What this does

The autoInstrument Vite plugin parses SvelteKit +page.ts / +layout.server.ts files to detect export const load or export function load declarations. It only reads the AST — never transforms or prints it — so recast's source-preserving round-trip feature is entirely unused.

This replaces the full recast + @babel/parser pipeline with acorn (Node.js's own parser) and @sveltejs/acorn-typescript (the actively maintained TypeScript plugin used by SvelteKit itself).

Changes

Delete recastTypescriptParser.ts — the 91-line Babel parser config with 20+ plugin declarations is no longer needed
Replace recast.parse() with acorn.Parser.extend(tsPlugin()).parse() in autoInstrument.ts
Fix AST node type: StringLiteral (Babel-specific) → Literal (ESTree standard) for string literal export detection
Fix AST structure: ast.program (recast's File wrapper) → direct ast (acorn returns Program)
Improve error handling: try/catch around parse (acorn throws SyntaxError) replaces dubious null check
Improve type safety: remove unsafe double-cast (specifier.exported as unknown as t.StringLiteral) in favor of proper discriminant narrowing

Dependency reduction

Removed: recast, ast-types, esprima, source-map, tslib, tiny-invariant, @babel/parser, @babel/types, @babel/helper-string-parser, @babel/helper-validator-identifier

Added: acorn (already in dep tree via Vite/Rollup), @sveltejs/acorn-typescript (already in dep tree via @sveltejs/kit)

Net effect: ~8 fewer transitive dependencies, zero new packages for end users.

Why @sveltejs/acorn-typescript?

The original acorn-typescript package by TyrealHu hasn't been updated since January 2024 and has known unpatched bugs. The Svelte team's fork (@sveltejs/acorn-typescript) is actively maintained and is already a transitive dependency of every SvelteKit project.

All 69 existing tests pass without modification.

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

JPeer264

Thanks a lot for these changes, they look good. I actually want to see if the tests are going through and want to wait for a review from @Lms24 before we merge it.

Lms24

Hi, thanks for taking this on! This looks good to me! Once we drop support for SvelteKit <2.31.0, we can even get rid of these deps entirely but for now, the reduction from 8 to 2 deps already is a win.

heads-up: I adjusted the PR title to better reflect the type of change

packages/sveltekit/src/vite/autoInstrument.ts

Replaces recast and @babel/parser with acorn and @sveltejs/acorn-typescript for AST parsing in the auto-instrumentation plugin. Since the code only reads the AST (never transforms or prints), the full recast pipeline is unnecessary. Key changes: - Delete recastTypescriptParser.ts (no longer needed) - Use acorn Parser.extend(tsPlugin()) for TypeScript parsing - Use @sveltejs/acorn-typescript (actively maintained) instead of TyrealHu's abandoned acorn-typescript - Fix StringLiteral -> Literal node type check (ESTree compliance) - Fix ast.program -> ast (acorn returns Program directly) - Use try/catch for parse errors instead of null program check Removes ~8 transitive dependencies (recast, ast-types, esprima, source-map, tslib, tiny-invariant, @babel/parser, @babel/types). Ref: getsentry#19447 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- Fix import sort order in autoInstrument.ts (scoped package before unscoped) - Deduplicate @sveltejs/acorn-typescript entries in yarn.lock Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

JPeer264 · 2026-03-19T10:20:47Z

I just rebased the branch and deduplicated yarn.lock. Hopefully this can be merged then right after

github-actions · 2026-03-19T10:20:51Z

Semver Impact of This PR

🟢 Patch (bug fixes)

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New Features ✨

Deps

Bump mongodb-memory-server-global from 10.1.4 to 11.0.1 by dependabot in #19888
Bump stacktrace-parser from 0.1.10 to 0.1.11 by dependabot in #19887

Bug Fixes 🐛

(cloudflare) Forward ctx argument to Workflow.do user callback by Lms24 in #19891
(core) Return same value from startSpan as callback returns by s1gr1d in #19300
(deps) Bump socket.io-parser to 4.2.6 to fix CVE-2026-33151 by chargome in #19880
(nestjs) Add node to nest metadata by chargome in #19875
(serverless) Add node to metadata by nicohrubec in #19878

Internal Changes 🔧

(astro) Re-enable server island tracing e2e test in Astro 6 by Lms24 in #19872
(lint) Resolve oxlint warnings by isaacs in #19893
(node-integration-tests) Remove unnecessary file-type dependency by Lms24 in #19824

(sveltekit) Replace recast + @babel/parser with acorn by roli-lpci in #19533

_{🤖 This preview updates automatically when you update the PR.}

Lms24 · 2026-03-20T10:09:54Z

rebased once more after the last release. Gonna merge this afterwards!

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packages/sveltekit/package.json

Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>

This PR adds the external contributor to the CHANGELOG.md file, so that they are credited for their contribution. See #19533 Co-authored-by: Lms24 <8420481+Lms24@users.noreply.github.com>

roli-lpci mentioned this pull request Feb 26, 2026

Replace SDK package dependencies #19447

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roli-lpci force-pushed the chore/sveltekit-replace-recast-with-acorn branch from fa80627 to d9cb391 Compare February 26, 2026 18:23

roli-lpci mentioned this pull request Feb 26, 2026

chore(remix): replace yargs with native util.parseArgs #19532

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roli-lpci force-pushed the chore/sveltekit-replace-recast-with-acorn branch from d9cb391 to f54035e Compare March 1, 2026 04:11

Lms24 changed the title ~~chore(sveltekit): replace recast + @babel/parser with acorn~~ ref(sveltekit): Replace recast + @babel/parser with acorn Mar 2, 2026

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feat(deps): bump @babel/parser from 7.26.9 to 7.29.0 #19547

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roli-lpci force-pushed the chore/sveltekit-replace-recast-with-acorn branch from 470a2a7 to 2299bb2 Compare March 14, 2026 04:25

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roli-lpci and others added 3 commits March 19, 2026 11:19

fix: sort imports and deduplicate lockfile

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- Fix import sort order in autoInstrument.ts (scoped package before unscoped) - Deduplicate @sveltejs/acorn-typescript entries in yarn.lock Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

chore: Dedupe yarn.lock

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JPeer264 force-pushed the chore/sveltekit-replace-recast-with-acorn branch from 2299bb2 to 87f80a4 Compare March 19, 2026 10:20

Merge branch 'develop' into chore/sveltekit-replace-recast-with-acorn

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Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>

Lms24 enabled auto-merge (squash) March 20, 2026 11:11

Lms24 merged commit c1a7b22 into getsentry:develop Mar 20, 2026
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chore: Add external contributor to CHANGELOG.md #19909

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ref(sveltekit): Replace recast + @babel/parser with acorn#19533

ref(sveltekit): Replace recast + @babel/parser with acorn#19533
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roli-lpci commented Feb 26, 2026

What this does

Changes

Dependency reduction

Why @sveltejs/acorn-typescript?

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