fix(compiler-sfc): check lang before attempt to compile script

[alex-snezhko]

close #8368

Move the logic to avoid compilation if the script is not JS or TS before the file is actually attempted to be compiled.

I tested with a similar example to the reproduction link given in the issue:

App.vue:

<script lang="coffee">
export default
  data: ->
    count: 0

  methods:
    increment: ->
      @count += 1
</script>

<template>
  <button type="button" @click="increment">Count: {{ count }}</button>
</template>

vite.config.js:

import { defineConfig } from 'vite'
import vue from '@vitejs/plugin-vue'
import CoffeeScript from "coffeescript"
import * as sfcCompiler from "<locally built SFC compiler path>"

export default defineConfig({
  plugins: [
    vue({ compiler: sfcCompiler }),
    {
      name: 'coffee_compile',
      transform: (src, id) => {
        // compile coffee files to js
        if (/\.coffee$/.test(id)) {
          const { js, sourceMap } = CoffeeScript.compile(src, { sourceMap: true })
          return { code: js, map: sourceMap }
        }
      }
    }
  ],
})

Builds and functions as expected:

Summary by CodeRabbit

• New Features
  • Improved handling of script blocks with unrecognized language attributes, ensuring unsupported languages are preserved as-is and not processed.
• Bug Fixes
  • Prevented unnecessary processing of non-JavaScript/TypeScript script blocks.
• Refactor
  • Centralized language detection logic for scripts, streamlining checks for JavaScript and TypeScript.
• Tests
  • Added a test to verify correct behavior with unsupported script languages.

[coderabbitai[bot]]

Walkthrough

Refactors script language detection to use new isJS/isTS utilities, defers creating ScriptCompileContext until JS/TS scripts are detected, adds a test ensuring <script lang="coffee"> is returned unchanged with no AST, and centralizes language checks in script context utilities.

Changes

Cohort / File(s)	Change Summary
New test packages/compiler-sfc/__tests__/compileScript.spec.ts	Added test verifying <script lang="coffee"> is preserved verbatim, lang retained, and no scriptAst produced.
Language utils packages/compiler-sfc/src/script/utils.ts	Added isJS(...langs) and isTS(...langs) helpers to centralize JS/TS language detection.
Compile entry packages/compiler-sfc/src/compileScript.ts	Use isJS/isTS to compute isJSOrTS; early-return original script when not JS/TS; defer creating ScriptCompileContext until JS/TS detected.
Script context packages/compiler-sfc/src/script/context.ts	Replaced inline language checks with calls to isJS/isTS when initializing isJS/isTS flags.
Normal script processing packages/compiler-sfc/src/script/normalScript.ts	Removed prior inline early-return for non-JS/TS scripts so processing path inside the try block executes for all script languages.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant compileScript
    participant utils as isJS/isTS
    participant Response

    Caller->>compileScript: compile SFC (has <script lang="coffee">)
    compileScript->>utils: isJS(lang), isTS(lang)
    utils-->>compileScript: false / false
    compileScript-->>Response: return original script block (lang="coffee", no scriptAst)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Assessment against linked issues

Objective	Addressed	Explanation
Preserve and do not parse/transform custom script languages (e.g., CoffeeScript) in SFCs (#8368)	✅
Ensure compileScript returns the original script for unsupported languages, avoiding syntax errors (#8368)	✅
Add/verify tests for correct handling of non-JS/TS script blocks (#8368)	✅

Out-of-scope changes

Code Change	Explanation
Removal of early-return for non-JS/TS in normalScript processing (packages/compiler-sfc/src/script/normalScript.ts)	The linked issue expects unsupported languages to be returned early; allowing non-JS/TS scripts to proceed into the try processing path is not required by the issue and may change downstream behavior (not clearly tied to the stated objectives).

Possibly related PRs

• vuejs/core#13194 — Similar adjustments around early language checks and deferring ScriptCompileContext creation; strong code-level overlap.

Suggested reviewers

• LittleSound
• Doctor-wu

Poem

"I nibble code and hop with glee,
A coffee script stays safe with me.
No parse, no AST to make a fuss,
I keep your lang as written thus.
Hoppity-hop — preserved and free!"

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Reviewing files that changed from the base of the PR and between 99b49cf622fae7299fcaca4aa4bd37f16ed07bba and 80c40b38af9147b85c265390d2e06c289aec1120.

📒 Files selected for processing (1)

• packages/compiler-sfc/src/compileScript.ts (3 hunks)

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/compiler-sfc/src/compileScript.ts

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commit: 80c40b3

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB	38.5 kB	34.7 kB
vue.global.prod.js	159 kB	58.6 kB	52.2 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB	18.2 kB	16.7 kB
createApp	54.6 kB	21.3 kB	19.4 kB
createSSRApp	58.9 kB	23 kB	21 kB
defineCustomElement	59.6 kB	22.8 kB	20.8 kB
overall	68.7 kB	26.4 kB	24.1 kB

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Name	Link
🔨 Latest commit	99b49cf622fae7299fcaca4aa4bd37f16ed07bba
🔍 Latest deploy log	https://app.netlify.com/projects/vue-sfc-playground/deploys/68b6b6e28ce87c000847ccb4

[edison1105]

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