Investigation: Why vue-tsc ignores .vue files under Deno (denoland/deno#30977)

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Why vue-tsc ignores .vue files under Deno

Investigation of denoland/deno#30977

Summary

vue-tsc -b under Deno silently ignores .vue files because Deno's CJS require system bypasses fs.readFileSync, which @volar/typescript relies on to inject its TypeScript compiler patches.

How vue-tsc works

vue-tsc uses @volar/typescript's runTsc() function. The patching mechanism is:

1. Monkey-patch fs.readFileSync to intercept reads of TypeScript's tsc.js
2. Call require(tscPath) which triggers the patched readFileSync
3. The intercepted read returns a transformed version of tsc.js with:
   • .vue added to supportedTSExtensions, supportedJSExtensions, allSupportedExtensions
   • createProgram replaced with Volar's proxyCreateProgram wrapper
   • changeExtension patched to handle .vue extensions
4. The modified tsc.js runs with full Vue SFC support

The relevant code from @volar/typescript/lib/quickstart/runTsc.ts:

const readFileSync = fs.readFileSync;

(fs as any).readFileSync = (...args: any[]) => {
  if (args[0] === tscPath) {
    let tsc = (readFileSync as any)(...args) as string;
    return transformTscContent(tsc, proxyApiPath, extraSupportedExtensions, ...);
  }
  return (readFileSync as any)(...args);
};

try {
  return require(tscPath);  // This triggers the patched readFileSync
} finally {
  (fs as any).readFileSync = readFileSync;
  delete require.cache[tscPath];
}

The root cause

Deno's CJS require system reads files using op_require_read_file -- a direct Rust op that bypasses the node:fs module entirely.

From ext/node/polyfills/01_require.js:

function loadMaybeCjs(module, filename) {
  const content = op_require_read_file(filename);  // Direct Rust op, NOT fs.readFileSync!
  const format = op_require_is_maybe_cjs(filename) ? undefined : "module";
  module._compile(content, filename, format);
}

When require(tscPath) is called inside runTsc:

• Node.js: reads tsc.js via fs.readFileSync -> intercepted by Volar's monkey-patch -> returns transformed source with Vue support -> vue-tsc works
• Deno: reads tsc.js via op_require_read_file (Rust op) -> monkey-patch is completely bypassed -> vanilla tsc.js is loaded -> .vue files ignored

Impact

This doesn't just affect vue-tsc. Any tool that relies on monkey-patching fs.readFileSync to intercept require() file reads will be broken under Deno. This is a fundamental incompatibility in how Deno's CJS loader reads source files.

Possible fixes

1. Use fs.readFileSync in the CJS loader: Change op_require_read_file calls in Module._extensions handlers to go through the node:fs polyfill instead, so monkey-patches are respected. This would match Node.js behavior but could have performance implications.

2. Add a hook point: Provide a way for Module._extensions handlers to use a customizable file reader that defaults to the fast Rust op but can be overridden.

3. Detect the specific pattern: Check if fs.readFileSync has been monkey-patched and fall back to using it when loading CJS modules (similar to how wrapSafe() already detects Module.wrap patches via the patched flag).