diff --git a/lib/internal/main/worker_thread.js b/lib/internal/main/worker_thread.js
index 62a4bb1b7dd77e..94d0e613e8ce38 100644
--- a/lib/internal/main/worker_thread.js
+++ b/lib/internal/main/worker_thread.js
@@ -10,30 +10,118 @@ const {
 } = require('internal/bootstrap/pre_execution');
 
 const {
-  getEnvMessagePort,
-  threadId
+  threadId,
+  getEnvMessagePort
 } = internalBinding('worker');
 
 const {
-  createMessageHandler,
-  createWorkerFatalExeception
-} = require('internal/process/worker_thread_only');
+  messageTypes: {
+    // Messages that may be received by workers
+    LOAD_SCRIPT,
+    // Messages that may be posted from workers
+    UP_AND_RUNNING,
+    ERROR_MESSAGE,
+    COULD_NOT_SERIALIZE_ERROR,
+    // Messages that may be either received or posted
+    STDIO_PAYLOAD,
+    STDIO_WANTS_MORE_DATA,
+  },
+  kStdioWantsMoreDataCallback
+} = require('internal/worker/io');
 
+const {
+  fatalException: originalFatalException
+} = require('internal/process/execution');
+
+const publicWorker = require('worker_threads');
 const debug = require('util').debuglog('worker');
-debug(`[${threadId}] is setting up worker child environment`);
 
-function prepareUserCodeExecution() {
-  initializeClusterIPC();
-  initializeESMLoader();
-  loadPreloadModules();
-}
+debug(`[${threadId}] is setting up worker child environment`);
 
 // Set up the message port and start listening
 const port = getEnvMessagePort();
-port.on('message', createMessageHandler(port, prepareUserCodeExecution));
-port.start();
+
+port.on('message', (message) => {
+  if (message.type === LOAD_SCRIPT) {
+    const {
+      filename,
+      doEval,
+      workerData,
+      publicPort,
+      manifestSrc,
+      manifestURL,
+      hasStdin
+    } = message;
+    if (manifestSrc) {
+      require('internal/process/policy').setup(manifestSrc, manifestURL);
+    }
+    initializeClusterIPC();
+    initializeESMLoader();
+    loadPreloadModules();
+    publicWorker.parentPort = publicPort;
+    publicWorker.workerData = workerData;
+
+    if (!hasStdin)
+      process.stdin.push(null);
+
+    debug(`[${threadId}] starts worker script ${filename} ` +
+          `(eval = ${eval}) at cwd = ${process.cwd()}`);
+    port.unref();
+    port.postMessage({ type: UP_AND_RUNNING });
+    if (doEval) {
+      const { evalScript } = require('internal/process/execution');
+      evalScript('[worker eval]', filename);
+    } else {
+      process.argv[1] = filename; // script filename
+      require('module').runMain();
+    }
+    return;
+  } else if (message.type === STDIO_PAYLOAD) {
+    const { stream, chunk, encoding } = message;
+    process[stream].push(chunk, encoding);
+    return;
+  } else if (message.type === STDIO_WANTS_MORE_DATA) {
+    const { stream } = message;
+    process[stream][kStdioWantsMoreDataCallback]();
+    return;
+  }
+
+  require('assert').fail(`Unknown worker message type ${message.type}`);
+});
 
 // Overwrite fatalException
-process._fatalException = createWorkerFatalExeception(port);
+process._fatalException = (error) => {
+  debug(`[${threadId}] gets fatal exception`);
+  let caught = false;
+  try {
+    caught = originalFatalException.call(this, error);
+  } catch (e) {
+    error = e;
+  }
+  debug(`[${threadId}] fatal exception caught = ${caught}`);
+
+  if (!caught) {
+    let serialized;
+    try {
+      const { serializeError } = require('internal/error-serdes');
+      serialized = serializeError(error);
+    } catch {}
+    debug(`[${threadId}] fatal exception serialized = ${!!serialized}`);
+    if (serialized)
+      port.postMessage({
+        type: ERROR_MESSAGE,
+        error: serialized
+      });
+    else
+      port.postMessage({ type: COULD_NOT_SERIALIZE_ERROR });
+
+    const { clearAsyncIdStack } = require('internal/async_hooks');
+    clearAsyncIdStack();
+
+    process.exit();
+  }
+};
 
 markBootstrapComplete();
+
+port.start();
diff --git a/lib/internal/process/worker_thread_only.js b/lib/internal/process/worker_thread_only.js
index 2171d2b586a4ca..f05d5e932bebf2 100644
--- a/lib/internal/process/worker_thread_only.js
+++ b/lib/internal/process/worker_thread_only.js
@@ -3,13 +3,10 @@
 // This file contains process bootstrappers that can only be
 // run in the worker thread.
 const {
-  getEnvMessagePort,
-  threadId
+  getEnvMessagePort
 } = internalBinding('worker');
 
 const {
-  messageTypes,
-  kStdioWantsMoreDataCallback,
   kWaitingStreams,
   ReadableWorkerStdio,
   WritableWorkerStdio
@@ -18,15 +15,6 @@ const {
 const {
   codes: { ERR_WORKER_UNSUPPORTED_OPERATION }
 } = require('internal/errors');
-
-let debuglog;
-function debug(...args) {
-  if (!debuglog) {
-    debuglog = require('util').debuglog('worker');
-  }
-  return debuglog(...args);
-}
-
 const workerStdio = {};
 
 function initializeWorkerStdio() {
@@ -43,97 +31,6 @@ function initializeWorkerStdio() {
   };
 }
 
-function createMessageHandler(port, prepareUserCodeExecution) {
-  const publicWorker = require('worker_threads');
-
-  return function(message) {
-    if (message.type === messageTypes.LOAD_SCRIPT) {
-      const {
-        filename,
-        doEval,
-        workerData,
-        publicPort,
-        manifestSrc,
-        manifestURL,
-        hasStdin
-      } = message;
-      if (manifestSrc) {
-        require('internal/process/policy').setup(manifestSrc, manifestURL);
-      }
-      prepareUserCodeExecution();
-      publicWorker.parentPort = publicPort;
-      publicWorker.workerData = workerData;
-
-      if (!hasStdin)
-        workerStdio.stdin.push(null);
-
-      debug(`[${threadId}] starts worker script ${filename} ` +
-            `(eval = ${eval}) at cwd = ${process.cwd()}`);
-      port.unref();
-      port.postMessage({ type: messageTypes.UP_AND_RUNNING });
-      if (doEval) {
-        const { evalScript } = require('internal/process/execution');
-        evalScript('[worker eval]', filename);
-      } else {
-        process.argv[1] = filename; // script filename
-        require('module').runMain();
-      }
-      return;
-    } else if (message.type === messageTypes.STDIO_PAYLOAD) {
-      const { stream, chunk, encoding } = message;
-      workerStdio[stream].push(chunk, encoding);
-      return;
-    } else if (message.type === messageTypes.STDIO_WANTS_MORE_DATA) {
-      const { stream } = message;
-      workerStdio[stream][kStdioWantsMoreDataCallback]();
-      return;
-    }
-
-    require('assert').fail(`Unknown worker message type ${message.type}`);
-  };
-}
-
-// XXX(joyeecheung): this has to be returned as an anonymous function
-// wrapped in a closure, see the comment of the original
-// process._fatalException in lib/internal/process/execution.js
-function createWorkerFatalExeception(port) {
-  const {
-    fatalException: originalFatalException
-  } = require('internal/process/execution');
-
-  return (error) => {
-    debug(`[${threadId}] gets fatal exception`);
-    let caught = false;
-    try {
-      caught = originalFatalException.call(this, error);
-    } catch (e) {
-      error = e;
-    }
-    debug(`[${threadId}] fatal exception caught = ${caught}`);
-
-    if (!caught) {
-      let serialized;
-      try {
-        const { serializeError } = require('internal/error-serdes');
-        serialized = serializeError(error);
-      } catch {}
-      debug(`[${threadId}] fatal exception serialized = ${!!serialized}`);
-      if (serialized)
-        port.postMessage({
-          type: messageTypes.ERROR_MESSAGE,
-          error: serialized
-        });
-      else
-        port.postMessage({ type: messageTypes.COULD_NOT_SERIALIZE_ERROR });
-
-      const { clearAsyncIdStack } = require('internal/async_hooks');
-      clearAsyncIdStack();
-
-      process.exit();
-    }
-  };
-}
-
 // The execution of this function itself should not cause any side effects.
 function wrapProcessMethods(binding) {
   function umask(mask) {
@@ -150,7 +47,5 @@ function wrapProcessMethods(binding) {
 
 module.exports = {
   initializeWorkerStdio,
-  createMessageHandler,
-  createWorkerFatalExeception,
   wrapProcessMethods
 };