Tern v1.16.0

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Decisions

0026. Svelte's compiler in Go

2 min read Edit on GitHub

  • Status: Proposed
  • Date: 2026-11-12

#Context

tern dev and tern build need Node, for Vite and for Svelte's compiler, which is JavaScript. The app that's shipped doesn't: it runs, and since v1.13 mostly renders, without Node or a JavaScript engine. A build without Node would need Svelte's compiler without Node first, and Tern has a way to get it: native rendering compiles JavaScript to Go (ADR 0011), and it could compile the compiler.

#The spike

We compiled svelte/compiler (Svelte 5.57) to Go with the native compiler and compiled every .svelte file in the repository with it, 445 components from Tern UI, the examples and tern.land, comparing the output with Node's byte for byte.

  • Correctness: all 445 match, for server and for client output.
  • Speed: 3.1 s for the 445, cold, against about 0.5 s of compiling in Node; about 3 ms a component warm, against about 1 ms. Making the realm takes 90 ms. Slower than V8's JIT, and fast enough for builds and for recompiling a file on save.
  • Size: the compiled compiler is 6.8 MB of code, on a CLI of 26 MB.
  • The input: the compiler and its dependencies were compiled as one bundled module. Recorded module by module, as the Vite plugin records an app, three of its dependencies are CommonJS (aria-query, axobject-query and a UMD build of @jridgewell/resolve-uri), which the native compiler doesn't support.

It found three bugs in native rendering, fixed with conformance cases, which apps could hit too:

  • var declared in a loop or block at a module's top level crashed the compiler.
  • \p{ID_Start} and \p{ID_Continue} in regular expressions failed to compile.
  • A sticky or global regular expression after non-ASCII text started at the wrong place: lastIndex was given to the regexp engine as a rune index where it takes a byte offset.

#Proposal

A tern build that needs only Go, for apps without Vite plugins:

  • The compiler is compiled to Go once per Svelte version, when Tern is released, and ships in the CLI.
  • Bundling the client, TypeScript and CSS is esbuild's, which is Go.
  • npm packages still come from node_modules, installed as today; resolving them is esbuild's.
  • Vite stays the default, and the path for apps with Vite plugins, such as Tailwind's, and for tern dev's hot reloading.

Before that: the build has to match Vite's output closely enough (chunks, the asset manifest, CSS order), which is the larger part of the work; and CommonJS modules need compiling, for the compiler's dependencies recorded as they are and for apps' own.

#Consequences

  • If it's built, CI and Docker builds of most apps need Go only, and Tern controls its build's speed and versions.
  • Two build paths to keep matching, with tests that build the examples both ways and compare.
  • The fixes above help native rendering now, whether or not the build follows.