diff --git a/src/library_getvalue.js b/src/library_getvalue.js
index 26a4bd014aa53..061b797c83a2d 100644
--- a/src/library_getvalue.js
+++ b/src/library_getvalue.js
@@ -62,7 +62,7 @@ var LibraryMemOps = {
 
 #if SAFE_HEAP
   // The same as the above two functions, but known to the safeHeap pass
-  // in tools/acorn-optimizer.js.  The heap accesses within these two
+  // in tools/acorn-optimizer.mjs.  The heap accesses within these two
   // functions will *not* get re-written.
   // Note that we do not use the alias mechanism here since we need separate
   // instances of above setValueImpl/getValueImpl functions.
diff --git a/test/test_other.py b/test/test_other.py
index e0c9954641f8b..c71470fc8ec3f 100644
--- a/test/test_other.py
+++ b/test/test_other.py
@@ -2617,7 +2617,7 @@ def test_js_optimizer(self, input, passes):
     input = test_file(input)
     expected_file = os.path.splitext(input)[0] + '-output.js'
     # test calling optimizer
-    js = self.run_process(config.NODE_JS + [path_from_root('tools/acorn-optimizer.js'), input] + passes, stdin=PIPE, stdout=PIPE).stdout
+    js = self.run_process(config.NODE_JS + [path_from_root('tools/acorn-optimizer.mjs'), input] + passes, stdin=PIPE, stdout=PIPE).stdout
     if common.EMTEST_REBASELINE:
       write_file(expected_file, js)
     else:
diff --git a/third_party/terser/terser.js b/third_party/terser/terser.js
index d22224a6aa239..d64f71ea6b4c7 100644
--- a/third_party/terser/terser.js
+++ b/third_party/terser/terser.js
@@ -6706,7 +6706,7 @@ const _NOINLINE   = 0b00000100;
 const _KEY        = 0b00001000;
 const _MANGLEPROP = 0b00010000;
 
-// XXX Emscripten: export TreeWalker for walking through AST in acorn-optimizer.js.
+// XXX Emscripten: export TreeWalker for walking through AST in acorn-optimizer.mjs.
 exports.TreeWalker = TreeWalker;
 
 /***********************************************************************
diff --git a/tools/acorn-optimizer.js b/tools/acorn-optimizer.mjs
similarity index 99%
rename from tools/acorn-optimizer.js
rename to tools/acorn-optimizer.mjs
index 4edaec54ba5ec..c62e29f066665 100755
--- a/tools/acorn-optimizer.js
+++ b/tools/acorn-optimizer.mjs
@@ -1,10 +1,8 @@
 #!/usr/bin/env node
 
-'use strict';
-
-const acorn = require('acorn');
-const terser = require('../third_party/terser/terser');
-const fs = require('fs');
+import * as acorn from 'acorn';
+import * as terser from '../third_party/terser/terser.js';
+import * as fs from 'fs';
 
 // Utilities
 
diff --git a/tools/building.py b/tools/building.py
index 555396eeedd4a..96b4928fca0ed 100644
--- a/tools/building.py
+++ b/tools/building.py
@@ -349,7 +349,7 @@ def js_optimizer(filename, passes):
 
 # run JS optimizer on some JS, ignoring asm.js contents if any - just run on it all
 def acorn_optimizer(filename, passes, extra_info=None, return_output=False):
-  optimizer = path_from_root('tools/acorn-optimizer.js')
+  optimizer = path_from_root('tools/acorn-optimizer.mjs')
   original_filename = filename
   if extra_info is not None:
     temp_files = shared.get_temp_files()
@@ -579,7 +579,7 @@ def closure_compiler(filename, advanced=True, extra_closure_args=None):
     CLOSURE_EXTERNS += [path_from_root('src/closure-externs/minimal_runtime_worker_externs.js')]
 
   args = ['--compilation_level', 'ADVANCED_OPTIMIZATIONS' if advanced else 'SIMPLE_OPTIMIZATIONS']
-  # Keep in sync with ecmaVersion in tools/acorn-optimizer.js
+  # Keep in sync with ecmaVersion in tools/acorn-optimizer.mjs
   args += ['--language_in', 'ECMASCRIPT_2021']
   # Tell closure not to do any transpiling or inject any polyfills.
   # At some point we may want to look into using this as way to convert to ES5 but
diff --git a/tools/js_optimizer.py b/tools/js_optimizer.py
index b532ec70fe2ca..73eb0956e975f 100755
--- a/tools/js_optimizer.py
+++ b/tools/js_optimizer.py
@@ -22,7 +22,7 @@
 temp_files = shared.get_temp_files()
 
 
-ACORN_OPTIMIZER = path_from_root('tools/acorn-optimizer.js')
+ACORN_OPTIMIZER = path_from_root('tools/acorn-optimizer.mjs')
 
 NUM_CHUNKS_PER_CORE = 3
 MIN_CHUNK_SIZE = int(os.environ.get('EMCC_JSOPT_MIN_CHUNK_SIZE') or 512 * 1024) # configuring this is just for debugging purposes
@@ -65,7 +65,7 @@ def split_funcs(js):
 
 class Minifier:
   """minification support. We calculate minification of
-  globals here, then pass that into the parallel acorn-optimizer.js runners which
+  globals here, then pass that into the parallel acorn-optimizer.mjs runners which
   perform minification of locals.
   """
 
@@ -75,7 +75,7 @@ def __init__(self, js):
     self.profiling_funcs = False
 
   def minify_shell(self, shell, minify_whitespace):
-    # Run through acorn-optimizer.js to find and minify the global symbols
+    # Run through acorn-optimizer.mjs to find and minify the global symbols
     # We send it the globals, which it parses at the proper time. JS decides how
     # to minify all global names, we receive a dictionary back, which is then
     # used by the function processors