watch: re-expose watch flags via execArgv

lib/internal/main/watch_mode.js

@@ -7,6 +7,7 @@ const {
   ArrayPrototypePush,
   ArrayPrototypePushApply,
   ArrayPrototypeSlice,
+  JSONStringify,
   StringPrototypeStartsWith,
 } = primordials;
 
@@ -44,18 +45,22 @@ const kCommand = ArrayPrototypeSlice(process.argv, 1);
 const kCommandStr = inspect(ArrayPrototypeJoin(kCommand, ' '));
 
 const argsWithoutWatchOptions = [];
+const removedWatchFlags = [];
 const argsFromBinding = getOptionsAsFlagsFromBinding();
 for (let i = 0; i < argsFromBinding.length; i++) {
   const arg = argsFromBinding[i];
   if (StringPrototypeStartsWith(arg, '--watch=')) {
+    ArrayPrototypePush(removedWatchFlags, arg);
     continue;
   }
   if (arg === '--watch') {
+    ArrayPrototypePush(removedWatchFlags, arg);
     const nextArg = argsFromBinding[i + 1];
     if (nextArg && nextArg[0] !== '-') {
       // If `--watch` doesn't include `=` and the next
       // argument is not a flag then it is interpreted as
       // the watch argument, so we need to skip that as well
+      ArrayPrototypePush(removedWatchFlags, nextArg);
       i++;
     }
     continue;
@@ -66,7 +71,14 @@ for (let i = 0; i < argsFromBinding.length; i++) {
       // if --watch-path doesn't include `=` it means
       // that the next arg is the target path, so we
       // need to skip that as well
-      i++;
+      ArrayPrototypePush(removedWatchFlags, arg);
+      const nextArg = argsFromBinding[i + 1];
+      if (nextArg) {
+        ArrayPrototypePush(removedWatchFlags, nextArg);
+        i++;
+      }
+    } else {
+      ArrayPrototypePush(removedWatchFlags, arg);
     }
     continue;
   }
@@ -95,12 +107,16 @@ let exited;
 function start() {
   exited = false;
   const stdio = kShouldFilterModules ? ['inherit', 'inherit', 'inherit', 'ipc'] : 'inherit';
+  const env = {
+    ...process.env,
+    WATCH_REPORT_DEPENDENCIES: '1',
+  };
+  if (removedWatchFlags.length > 0) {
+    env.NODE_WATCH_ARGS = JSONStringify(removedWatchFlags);
+  }
   child = spawn(process.execPath, argsWithoutWatchOptions, {
     stdio,
-    env: {
-      ...process.env,
-      WATCH_REPORT_DEPENDENCIES: '1',
-    },
+    env,
   });
   watcher.watchChildProcessModules(child);
   if (kEnvFiles.length > 0) {

lib/internal/process/pre_execution.js

@@ -1,14 +1,17 @@
 'use strict';
 
 const {
+  ArrayIsArray,
   ArrayPrototypeForEach,
+  ArrayPrototypeUnshiftApply,
   Date,
   DatePrototypeGetDate,
   DatePrototypeGetFullYear,
   DatePrototypeGetHours,
   DatePrototypeGetMinutes,
   DatePrototypeGetMonth,
   DatePrototypeGetSeconds,
+  JSONParse,
   NumberParseInt,
   ObjectDefineProperty,
   ObjectFreeze,
@@ -270,6 +273,19 @@ function patchProcessObject(expandArgv1) {
   process._exiting = false;
   process.argv[0] = process.execPath;
 
+  const watchArgsFromLauncher = process.env.NODE_WATCH_ARGS;
+  if (watchArgsFromLauncher !== undefined) {
+    delete process.env.NODE_WATCH_ARGS;
+    try {
+      const parsed = JSONParse(watchArgsFromLauncher);
+      if (ArrayIsArray(parsed) && parsed.length > 0) {
+        ArrayPrototypeUnshiftApply(process.execArgv, parsed);
+      }
+    } catch {
+      // Ignore malformed payloads.
+    }
+  }
+
   /** @type {string} */
   let mainEntry;
   // If requested, update process.argv[1] to replace whatever the user provided with the resolved absolute file path of

test/sequential/test-watch-mode-watch-flags.mjs

@@ -94,4 +94,52 @@ describe('watch mode - watch flags', { concurrency: !process.env.TEST_PARALLEL,
       `Completed running ${inspect(file)}. Waiting for file changes before restarting...`,
     ]);
   });
+
+  it('exposes watch flags through process.execArgv inside the watched script', async () => {
+    const projectDir = tmpdir.resolve('project-watch-exec-argv');
+    mkdirSync(projectDir);
+
+    const file = createTmpFile(`
+      console.log(JSON.stringify(process.execArgv));
+    `, '.js', projectDir);
+    const watchPath = path.join(projectDir, 'template.html');
+    writeFileSync(watchPath, '');
+
+    async function assertExecArgv(args, expectedSubsequences) {
+      const { stdout, stderr } = await runNode({
+        args, options: { cwd: projectDir }
+      });
+
+      assert.strictEqual(stderr, '');
+
+      const execArgvLine = stdout[0];
+      const execArgv = JSON.parse(execArgvLine);
+      assert.ok(Array.isArray(execArgv));
+      const matched = expectedSubsequences.some((expectedSeq) => {
+        for (let i = 0; i <= execArgv.length - expectedSeq.length; i++) {
+          let ok = true;
+          for (let j = 0; j < expectedSeq.length; j++) {
+            if (execArgv[i + j] !== expectedSeq[j]) {
+              ok = false;
+              break;
+            }
+          }
+          if (ok) return true;
+        }
+        return false;
+      });
+      assert.ok(matched,
+                `execArgv (${execArgv}) does not contain any expected sequence (${expectedSubsequences.map((seq) => `[${seq}]`).join(', ')})`);
+      assert.match(stdout.at(-1), /^Completed running/);
+    }
+
+    await assertExecArgv(['--watch', file], [['--watch']]);
+    await assertExecArgv(['--watch-path=template.html', file], [['--watch-path=template.html']]);
+    await assertExecArgv(
+      ['--watch-path', 'template.html', file],
+      [
+        ['--watch-path', 'template.html'],
+        ['--watch-path=template.html'],
+      ]);
+  });
 });