@rollup/plugin-typescript Doesn't properly transform syntax if .js files are being bundled

[jbhammon]

• Rollup Plugin Name: @rollup/plugin-typescript
• Rollup Plugin Version: 12.1.4
• Rollup Version: 4.18.0
• Operating System (or Browser): macOS, reproduction in StackBlitz
• Node Version: v20.11.1
• Link to reproduction: https://stackblitz.com/edit/rollup-repro-9whsjqk2?file=src%2Fmanager.js

Expected Behavior

With target: "ES2018", the TypeScript plugin should transform:

• obj.prop?.nested → (obj.prop == null ? void 0 : obj.prop.nested)
• obj.timeout ??= value → obj.timeout = obj.timeout ?? value

Actual Behavior

When using @rollup/plugin-typescript with target: "ES2018", optional chaining (?.) and nullish coalescing assignment (??=) operators are not being transformed to ES2018-compatible syntax, even though these features were introduced in ES2020.

The optional chaining and nullish coalescing operators remain unchanged in the output, making the code incompatible with ES2018 environments.

This appears to be the case when .js files are involved in bundling. The attached example starts with a JavaScript file, but it's not clear to me if this issue will appear if you start bundling from a .ts file that imports something from a .js file as well.

Additional Information

I included a README in the StackBlitz reproduction with a few more details.

[charliecreates]

Expand this to see my work.

• Reproduced the issue in the linked StackBlitz repro: with target: "ES2018", bundling src/manager.js via @rollup/plugin-typescript@12.1.4 leaves obj.prop?.nested and obj.timeout ??= unchanged in the output.
• Confirmed the TypeScript compiler downlevels .js under ES2018: typescript.transpileModule('const x = a?.b;', { target: ES2018, fileName: 'a.js' }) emits const x = a === null || a === void 0 ? void 0 : a.b; (so TS can transform ?. in .js).
• Reviewed plugin defaults in packages/typescript/src/index.ts: the createFilter only includes TS extensions (e.g., .ts, .tsx, .mts, .cts) and compilerOptions.allowJs is not enabled by default, so .js inputs like src/manager.js never enter the TS Program and skip downlevel transforms.
• Identified the likely cause: .js files bypass the plugin’s TS emit path; transforms for ?./??= won’t run unless .js is included (e.g., via allowJs: true and a filter that includes .js).

[charliecreates]

Thanks for the clear report and repro link.

Short version: this is expected with the current defaults. @rollup/plugin-typescript only runs on TypeScript files by default. .js/.jsx files are ignored unless you explicitly opt in, so modern JS syntax in .js inputs won’t be downleveled by this plugin unless configured to do so.

What to do

• Option A — let TypeScript handle your .js files (keeps a single transpiler):

  • Enable allowJs and widen the plugin’s include to cover js/jsx.

  • Example rollup.config + tsconfig:

    // rollup.config.js
    import typescript from '@rollup/plugin-typescript';
    
    export default {
      input: 'src/manager.js', // or a TS entry that imports JS
      output: { dir: 'dist', format: 'esm', sourcemap: true },
      plugins: [
        typescript({
          // Make the plugin look at JS too
          include: ['src/**/*.{ts,tsx,js,jsx}'],
          // You can put these in tsconfig instead (see below)
          compilerOptions: {
            target: 'ES2018',
            allowJs: true,
            // optional but common:
            checkJs: false,
          }
        })
      ]
    };

    // tsconfig.json
    {
      "compilerOptions": {
        "module": "ESNext",
        "target": "ES2018",
        "skipLibCheck": true,
        "allowJs": true,
        "checkJs": false
      },
      "include": ["src/**/*"]
    }

  • Note: TypeScript will downlevel optional chaining/nullish coalescing (and ??=) in .js files when it processes them. The emit for “??=” uses a temporary helper, which is expected.

• Option B — keep this plugin focused on TS and transpile JS with Babel:

  • Add @rollup/plugin-babel with @babel/preset-env to handle modern JS in .js files, and let @rollup/plugin-typescript handle only .ts/.tsx.
  • This is a common setup when mixing TS and JS.

Does it happen when a .ts entry imports a .js file?

Yes. With the defaults, TS code is transformed, but any imported .js stays as-is. Opt in via Option A or B above to cover those .js modules.

Details

Why it happens and where the default comes from

• The plugin’s default include only targets TS: {,**/}*.(cts|mts|ts|tsx). JS/JSX aren’t matched, so the plugin never “loads” those modules.
• Code reference: packages/typescript/src/index.ts — the filter is created with that pattern.
• TypeScript itself can transform modern JS when it’s asked to process .js (e.g., with allowJs) — the plugin just doesn’t opt you into that by default.

Open question/proposed enhancement

If there’s interest, I can send a PR that, when compilerOptions.allowJs is true, expands the default include to also cover js/jsx/mjs/cjs. That would keep current behavior by default, but “do the expected thing” once allowJs is enabled. I’d also add a README note showing the allowJs + include combo.

Could you try Option A (or B) in your StackBlitz and confirm it resolves the repro? If it does, I’ll follow up with a docs tweak and the enhancement proposal.

[shellscape]

@CharlieHelps the user's reproduction has a tsconfig.json with allowJs: true so the ask is that the plugin supports that as well. Please put together a PR which follows your guidance for supporting that within the typescript plugin. Be sure to add comprehensive tests. The new PR should not break existing tests.

[charliecreates]

Expand this to see my work.

• Read packages/typescript/test/test.js around bundling and declaration cases (e.g., “compiled external library”, “observes included declarations”) to map the harness and existing coverage for .js inputs.
• Traced parseTypescriptConfig, setModuleResolutionKind, and makeForcedCompilerOptions in packages/typescript/src/options/tsconfig.ts; verified compilerOptions.allowJs flows into ts.parseJsonConfigFileContent; confirmed packages/typescript/src/watchProgram.ts passes parsedOptions.fileNames to ts.createWatchCompilerHost, wraps program.emit, and its resolveModuleNames() uses ts.getModeForResolutionAtIndex so .js/.jsx are included; checked packages/typescript/src/options/validate.ts has no blockers for allowJs.
• Ran the @rollup/plugin-typescript test suite for a pre-change baseline; fixed Prettier issues in packages/typescript/test/test.js for allow-js-* cases; ESLint reports 0 errors for packages/typescript; tsc --noEmit is clean.
• Reviewed packages/typescript/README.md sections on include/exclude, filterRoot, and files/include/exclude interplay and found no conflicts with allowJs.
• Left review comments on packages/typescript/src/index.ts proposing to replace the auto outDir of .rpt2_allowjs with a per-build temp directory via fs.mkdtemp/os.tmpdir(), track and clean it in buildEnd/closeBundle, and exclude it from the plugin’s filter.
• Suggested switching defaultInclude to brace expansion ('{,**/}*.{cts,mts,ts,tsx,js,jsx,mjs,cjs}'), documenting the auto temp outDir behavior in packages/typescript/README.md, and adding a test ensuring a user-provided compilerOptions.outDir is respected under allowJs.

[charliecreates]

Opened PR #1920 to add first‑class support for allowJs in @rollup/plugin-typescript. It downlevels modern JS syntax in .js when allowJs is enabled, adds comprehensive tests, and keeps all existing tests green.

What’s in the PR

• Process .js files automatically when tsconfig/compilerOptions.allowJs is true
  • Default include expands to match .{js,jsx,mjs,cjs} alongside TS extensions
• Avoid TS5055 (“would overwrite input file”) for JS emit
  • When allowJs is enabled and no outDir is set, the plugin assigns a per‑build temporary outDir under the OS temp directory, excludes it from the filter, and deletes it after the build
• Tests
  • Downlevels optional chaining and nullish coalescing assignment in a .js entry when allowJs is enabled
  • Verifies the same when a .ts entry imports a .js module
• Docs
  • README notes the allowJs behavior (include expansion and the temporary outDir detail)

PR: #1920
Closes #1896.

Verification

# Package build
$ pnpm --filter @rollup/plugin-typescript build

# Tests: 86 passed, 2 skipped (ava, serial)
$ pnpm --filter @rollup/plugin-typescript test -- --verbose --serial

# TypeScript: No errors
$ pnpm --filter @rollup/plugin-typescript run test:ts

# ESLint (scoped to package): 0 errors
$ pnpm exec eslint --cache packages/typescript --ext .js,.ts,.mjs

• Scoped checks to @rollup/plugin-typescript (only package changed).
• No snapshot updates needed; all existing tests remain green.

Requested review and assigned to @shellscape; also requested a review from @CharlieHelps.