diff --git a/packages/v2-es-membrane/_00_ecma_references/fixtures/NumberStringClass.mts b/packages/v2-es-membrane/_00_ecma_references/fixtures/NumberStringClass.ts
similarity index 100%
rename from packages/v2-es-membrane/_00_ecma_references/fixtures/NumberStringClass.mts
rename to packages/v2-es-membrane/_00_ecma_references/fixtures/NumberStringClass.ts
diff --git a/packages/v2-es-membrane/_00_ecma_references/fixtures/types/NumberStringType.d.mts b/packages/v2-es-membrane/_00_ecma_references/fixtures/types/NumberStringType.d.ts
similarity index 100%
rename from packages/v2-es-membrane/_00_ecma_references/fixtures/types/NumberStringType.d.mts
rename to packages/v2-es-membrane/_00_ecma_references/fixtures/types/NumberStringType.d.ts
diff --git a/packages/v2-es-membrane/_00_ecma_references/spec/argumentTruncation.mjs b/packages/v2-es-membrane/_00_ecma_references/spec/argumentTruncation.js
similarity index 100%
rename from packages/v2-es-membrane/_00_ecma_references/spec/argumentTruncation.mjs
rename to packages/v2-es-membrane/_00_ecma_references/spec/argumentTruncation.js
diff --git a/packages/v2-es-membrane/_00_ecma_references/spec/classDecorators.mts b/packages/v2-es-membrane/_00_ecma_references/spec/classDecorators.ts
similarity index 100%
rename from packages/v2-es-membrane/_00_ecma_references/spec/classDecorators.mts
rename to packages/v2-es-membrane/_00_ecma_references/spec/classDecorators.ts
diff --git a/packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.mts b/packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.ts
similarity index 97%
rename from packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.mts
rename to packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.ts
index 6bee1e1e..e2da4388 100644
--- a/packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.mts
+++ b/packages/v2-es-membrane/_00_ecma_references/spec/classMethodDecorators.ts
@@ -2,9 +2,9 @@
 
 import type {
   NumberStringType
-} from "../fixtures/types/NumberStringType.mjs";
+} from "../fixtures/types/NumberStringType.js";
 
-import NumberStringClass from "../fixtures/NumberStringClass.mjs";
+import NumberStringClass from "../fixtures/NumberStringClass.js";
 
 type ClassMethodDecorator<
   This extends object,
diff --git a/packages/v2-es-membrane/_00_ecma_references/spec/support/jasmine.json b/packages/v2-es-membrane/_00_ecma_references/spec/support/jasmine.json
index 39d5e0cd..7c0d55be 100644
--- a/packages/v2-es-membrane/_00_ecma_references/spec/support/jasmine.json
+++ b/packages/v2-es-membrane/_00_ecma_references/spec/support/jasmine.json
@@ -1,7 +1,6 @@
 {
   "spec_files": [
-    "./spec/**/*.mjs",
-    "./spec/**/*.mts",
+    "./spec/**/*.js",
     "./spec/**/*.ts"
   ],
   "stopSpecOnExpectationFailure": false
diff --git a/packages/v2-es-membrane/_01_stage_utilities/README.md b/packages/v2-es-membrane/_01_stage_utilities/README.md
index a732811b..1b453699 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/README.md
+++ b/packages/v2-es-membrane/_01_stage_utilities/README.md
@@ -4,27 +4,27 @@ This directory is an infrastructure grab-bag:  it provides common classes to sup
 
 ## Generic tools
 
-- [DefaultMap](./source/DefaultMap.mts) extends `Map` and `WeakMap`, to provide a `getDefault()` method where the user can set a value for a key if there isn't an existing value.
-- [PromiseTypes](./source/PromiseTypes.mts) is a set of utilities relating directly to basic promises.
+- [DefaultMap](./source/DefaultMap.ts) extends `Map` and `WeakMap`, to provide a `getDefault()` method where the user can set a value for a key if there isn't an existing value.
+- [PromiseTypes](./source/PromiseTypes.ts) is a set of utilities relating directly to basic promises.
   - `TimeoutPromise` for promises rejecting after a delay.  Useful with `Promise.race()` to ensure a task finishes in a certain length of time.
   - `SingletonPromise` for wrapping a Promise-returning method so it only runs once.
   - `PromiseAllParallel` and its sibling `PromiseAllSequence`, for mapping an array of objects into an array of promises, and then waiting for all the promises to resolve.
   - `PromiseDictionary` for converting a dictionary of promises (`{ [key in keyof T]: Promise<T[key]> }`) into a promise of a dictionary (`Promise<T>`).  See [the proposal-await-dictionary](https://github.com/tc39/proposal-await-dictionary) repository, which I originally started.
-- [PropertyKeySorter](./source/PropertyKeySorter.mts) is a simple utility for sorting:
+- [PropertyKeySorter](./source/PropertyKeySorter.ts) is a simple utility for sorting:
   1. strings by JavaScript's default sort ordering,
   2. symbols in the order the PropertyKeySorter sees them.
-- [ReplaceableValueMap](./source/ReplaceableValueMap.mts) lets you define a value you want to subsitute for another, and a shared object for that replaced value to use.  This is useful for decorators which want to have a _single_ replacement for an original value (i.e. the same class decorator run more than once won't subclass more than once).
-- [WeakRefSet](./source/WeakRefSet.mts) is basically a `Set<WeakRef<T>>`: it stores weak references and allows you to retrieve them later.
-- [SharedAssertSet](./source/SharedAssertSet.mts): assertion failures in one object, notifying other objects of the failure so all of them can shut down.  Membranes have several proxy handlers which the user should never see, so an assertion failure in one proxy handler can terminate an entire membrane.
-  - [types/assert.d.mts](./source/types/assert.d.mts) defines common assertion interfaces and types.
-- [RequiredInitializers](./source/RequiredInitializers.mts) is a very simple state machine for defining and resolving prerequisites, when you can't use constructors.  [See mix-in decorators](../_02_mixin_decorators/README.md).
-- [`maybeDefined.mts`](./source/maybeDefined.mts) provides some utilities for making sure we've filled in some fields.  This may not have been my best idea... but it works.
-- [types/Utility.d.mts](./source/types/Utility.d.mts) provides utility types, many of which I don't use but I thought I might need.
+- [ReplaceableValueMap](./source/ReplaceableValueMap.ts) lets you define a value you want to subsitute for another, and a shared object for that replaced value to use.  This is useful for decorators which want to have a _single_ replacement for an original value (i.e. the same class decorator run more than once won't subclass more than once).
+- [WeakRefSet](./source/WeakRefSet.ts) is basically a `Set<WeakRef<T>>`: it stores weak references and allows you to retrieve them later.
+- [SharedAssertSet](./source/SharedAssertSet.ts): assertion failures in one object, notifying other objects of the failure so all of them can shut down.  Membranes have several proxy handlers which the user should never see, so an assertion failure in one proxy handler can terminate an entire membrane.
+  - [types/assert.d.ts](./source/types/assert.d.ts) defines common assertion interfaces and types.
+- [RequiredInitializers](./source/RequiredInitializers.ts) is a very simple state machine for defining and resolving prerequisites, when you can't use constructors.  [See mix-in decorators](../_02_mixin_decorators/README.md).
+- [`maybeDefined.ts`](./source/maybeDefined.ts) provides some utilities for making sure we've filled in some fields.  This may not have been my best idea... but it works.
+- [types/Utility.d.ts](./source/types/Utility.d.ts) provides utility types, many of which I don't use but I thought I might need.
 
 ## Internal build tools
 
-- [AsyncSpecModules](./source/AsyncSpecModules.mts) is for Jasmine test specifications to load modules dynamically.  Think of this as wrapping [the import() function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import) in TypeScript.
-- [getTS_SourceFile.mts](./source/getTS_SourceFile.mts) uses [ts-morph](https://ts-morph.com) to parse TypeScript source files.
-- [SpyBase](./source/SpyBase.mts) provides a helper class for Jasmine spies.
-- [StateMachine_DFA](./source/stateMachines/dfa-states.mts) is a very bare-bones state machine implementation.
-- [holdsArgument](./source/gc/holdsArgument.mts) and [holdsReturn](./source/gc/holdsReturn.mts) are for Jasmine test specifications to use, testing whether a particular function holds a reference to an argument or its return value, respectively.  They use two NodeJS-specific functions, `gc()` and `setImmediatePromise()`, for tests involving garbage collection.
+- [AsyncSpecModules](./source/AsyncSpecModules.ts) is for Jasmine test specifications to load modules dynamically.  Think of this as wrapping [the import() function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import) in TypeScript.
+- [getTS_SourceFile.ts](./source/getTS_SourceFile.ts) uses [ts-morph](https://ts-morph.com) to parse TypeScript source files.
+- [SpyBase](./source/SpyBase.ts) provides a helper class for Jasmine spies.
+- [StateMachine_DFA](./source/stateMachines/dfa-states.ts) is a very bare-bones state machine implementation.
+- [holdsArgument](./source/gc/holdsArgument.ts) and [holdsReturn](./source/gc/holdsReturn.ts) are for Jasmine test specifications to use, testing whether a particular function holds a reference to an argument or its return value, respectively.  They use two NodeJS-specific functions, `gc()` and `setImmediatePromise()`, for tests involving garbage collection.
diff --git a/packages/v2-es-membrane/_01_stage_utilities/examples/run.mts b/packages/v2-es-membrane/_01_stage_utilities/examples/run.ts
similarity index 62%
rename from packages/v2-es-membrane/_01_stage_utilities/examples/run.mts
rename to packages/v2-es-membrane/_01_stage_utilities/examples/run.ts
index 3d105e06..bb8ac112 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/examples/run.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/examples/run.ts
@@ -1,4 +1,4 @@
-import { doubleArray } from "./source/AsyncModule.mjs";
+import { doubleArray } from "./source/AsyncModule.js";
 
 export default async function() : Promise<void> {
   console.log(await doubleArray.run());
diff --git a/packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.mts b/packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.ts
similarity index 76%
rename from packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.mts
rename to packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.ts
index 29b439a0..fee3b561 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/examples/source/AsyncModule.ts
@@ -1,11 +1,11 @@
 import {
   getModuleDefaultClassWithArgs,
   ModuleSourceDirectory,
-} from "../../source/AsyncSpecModules.mjs";
+} from "../../source/AsyncSpecModules.js";
 
 import type {
   DoubleArrayPromiseType,
-} from "./DoubleArrayPromise.mjs";
+} from "./DoubleArrayPromise.js";
 
 const moduleSource: ModuleSourceDirectory = {
   importMeta: import.meta,
@@ -16,7 +16,7 @@ const DoubleArrayPromise = await getModuleDefaultClassWithArgs<
   [number],
   DoubleArrayPromiseType
 >(
-  moduleSource, "DoubleArrayPromise.mjs"
+  moduleSource, "DoubleArrayPromise.js"
 );
 
 const doubleArray = new DoubleArrayPromise(100);
diff --git a/packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.mts b/packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.ts
similarity index 94%
rename from packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.mts
rename to packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.ts
index 39ea0f54..6d9772b3 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/examples/source/DoubleArrayPromise.ts
@@ -1,7 +1,7 @@
 import {
   PromiseAllSequence,
   SingletonPromise,
-} from "../../source/PromiseTypes.mjs";
+} from "../../source/PromiseTypes.js";
 
 export type DoubleArrayPromiseType = {
   run() : Promise<number[]>;
diff --git a/packages/v2-es-membrane/_01_stage_utilities/fixtures/NumberStringClass.mts b/packages/v2-es-membrane/_01_stage_utilities/fixtures/NumberStringClass.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/fixtures/NumberStringClass.mts
rename to packages/v2-es-membrane/_01_stage_utilities/fixtures/NumberStringClass.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/fixtures/types/NumberStringType.d.mts b/packages/v2-es-membrane/_01_stage_utilities/fixtures/types/NumberStringType.d.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/fixtures/types/NumberStringType.d.mts
rename to packages/v2-es-membrane/_01_stage_utilities/fixtures/types/NumberStringType.d.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/AsyncSpecModules.mts b/packages/v2-es-membrane/_01_stage_utilities/source/AsyncSpecModules.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/AsyncSpecModules.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/AsyncSpecModules.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/PromiseTypes.mts b/packages/v2-es-membrane/_01_stage_utilities/source/PromiseTypes.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/PromiseTypes.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/PromiseTypes.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/PropertyKeySorter.mts b/packages/v2-es-membrane/_01_stage_utilities/source/PropertyKeySorter.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/PropertyKeySorter.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/PropertyKeySorter.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/ReplaceableValueMap.mts b/packages/v2-es-membrane/_01_stage_utilities/source/ReplaceableValueMap.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/ReplaceableValueMap.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/ReplaceableValueMap.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/RequiredInitializers.mts b/packages/v2-es-membrane/_01_stage_utilities/source/RequiredInitializers.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/RequiredInitializers.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/RequiredInitializers.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/SpyBase.mts b/packages/v2-es-membrane/_01_stage_utilities/source/SpyBase.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/SpyBase.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/SpyBase.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.mts b/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.ts
similarity index 93%
rename from packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.ts
index f0ac91cb..00cbec94 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsArgument.ts
@@ -1,4 +1,4 @@
-import holdsReturn from "./holdsReturn.mjs";
+import holdsReturn from "./holdsReturn.js";
 
 /**
  * Report if a callback function holds references to any values.
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.mts b/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.ts
similarity index 94%
rename from packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.ts
index b787eb63..b344485b 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/source/gc/holdsReturn.ts
@@ -1,7 +1,7 @@
-import tryGarbageCollection from "./tryGarbageCollection.mjs";
+import tryGarbageCollection from "./tryGarbageCollection.js";
 import PromiseFinalizer, {
   type PromiseResolver
-} from "./promiseFinalizer.mjs";
+} from "./promiseFinalizer.js";
 
 type MaybeHoldReturn = () => object;
 
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/gc/promiseFinalizer.mts b/packages/v2-es-membrane/_01_stage_utilities/source/gc/promiseFinalizer.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/gc/promiseFinalizer.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/gc/promiseFinalizer.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/gc/tryGarbageCollection.mts b/packages/v2-es-membrane/_01_stage_utilities/source/gc/tryGarbageCollection.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/gc/tryGarbageCollection.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/gc/tryGarbageCollection.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.mts b/packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.ts
similarity index 97%
rename from packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.ts
index e59a0bfb..2257aa90 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/source/getTS_SourceFile.ts
@@ -10,7 +10,7 @@ import {
 import {
   ModuleSourceDirectory,
   pathToModule,
-} from "./AsyncSpecModules.mjs";
+} from "./AsyncSpecModules.js";
 
 const project = new Project({
   "compilerOptions": {
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/maybeDefined.mts b/packages/v2-es-membrane/_01_stage_utilities/source/maybeDefined.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/maybeDefined.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/maybeDefined.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/stateMachines/dfa-states.mts b/packages/v2-es-membrane/_01_stage_utilities/source/stateMachines/dfa-states.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/stateMachines/dfa-states.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/stateMachines/dfa-states.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.mts b/packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.ts
similarity index 94%
rename from packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.ts
index 9cd6eac2..2bac8ec2 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/source/types/ClassMethodDecoratorFunction.d.ts
@@ -2,7 +2,7 @@
 
 import type {
   MethodsOnlyType
-} from "./MethodsOnlyType.d.mts";
+} from "./MethodsOnlyType.js";
 
 export type ClassMethodDecoratorFunction<
   This extends MethodsOnlyType,
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/types/MethodsOnlyType.d.mts b/packages/v2-es-membrane/_01_stage_utilities/source/types/MethodsOnlyType.d.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/types/MethodsOnlyType.d.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/types/MethodsOnlyType.d.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/source/types/Utility.d.mts b/packages/v2-es-membrane/_01_stage_utilities/source/types/Utility.d.ts
similarity index 100%
rename from packages/v2-es-membrane/_01_stage_utilities/source/types/Utility.d.mts
rename to packages/v2-es-membrane/_01_stage_utilities/source/types/Utility.d.ts
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.ts
similarity index 75%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.ts
index d022c713..116fd5ce 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/AsyncSpecModules.ts
@@ -6,16 +6,16 @@ import {
   getModuleDefaultClass,
   getModuleDefaultClassWithArgs,
   getModulePart,
-} from "../source/AsyncSpecModules.mjs";
+} from "../source/AsyncSpecModules.js";
 
-import NumberStringClass from "../fixtures/NumberStringClass.mjs";
+import NumberStringClass from "../fixtures/NumberStringClass.js";
 import type {
   NumberStringType
-} from "../fixtures/types/NumberStringType.mjs";
+} from "../fixtures/types/NumberStringType.js";
 
 import type {
   DoubleArrayPromiseType,
-} from "../examples/source/DoubleArrayPromise.mjs";
+} from "../examples/source/DoubleArrayPromise.js";
 
 const stageDir: ModuleSourceDirectory = {
   importMeta: import.meta,
@@ -25,20 +25,20 @@ const stageDir: ModuleSourceDirectory = {
 describe("AsyncSpecModules", () => {
   it("pathToModule provides resolved paths", () => {
     expect(
-      pathToModule(stageDir, "spec/AsyncSpecModules.mts")
+      pathToModule(stageDir, "spec/AsyncSpecModules.ts")
     ).toBe(fileURLToPath(import.meta.url));
 
     expect(
       pathToModule({
         pathToDirectory: "#stage_utilities/spec",
         isAbsolutePath: true
-      }, "AsyncSpecModules.mts")
+      }, "AsyncSpecModules.ts")
     ).toBe(fileURLToPath(import.meta.url));
   });
 
   it("getModuleDefaultClass retrieves an exported module", async () => {
     await expectAsync(getModuleDefaultClass<NumberStringType>(
-      stageDir, "fixtures/NumberStringClass.mjs"
+      stageDir, "fixtures/NumberStringClass.js"
     )).toBeResolvedTo(NumberStringClass);
   });
 
@@ -48,7 +48,7 @@ describe("AsyncSpecModules", () => {
       DoubleArrayPromiseType
     >
     (
-      stageDir, "examples/source/DoubleArrayPromise.mjs"
+      stageDir, "examples/source/DoubleArrayPromise.js"
     );
     const doubleArray = new DoubleArrayPromise(100);
     expect((await doubleArray.run()).length).toBe(100);
@@ -59,7 +59,7 @@ describe("AsyncSpecModules", () => {
       "pathToModule", typeof pathToModule
     >
     (
-      stageDir, "source/AsyncSpecModules.mjs", "pathToModule"
+      stageDir, "source/AsyncSpecModules.js", "pathToModule"
     );
 
     expect(pathToModuleAsPart).toBe(pathToModule);
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.ts
similarity index 96%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.ts
index eeacb8ac..34353483 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/PromiseDictionary.ts
@@ -1,4 +1,4 @@
-import { PromiseDictionary } from "#stage_utilities/source/PromiseTypes.mjs";
+import { PromiseDictionary } from "#stage_utilities/source/PromiseTypes.js";
 
 it("PromiseDictionary resolves", async () => {
   const THREE = Symbol("three");
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.ts
similarity index 95%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.ts
index 71554c57..a61a15ab 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/PromiseTypes/SingletonPromise.ts
@@ -1,6 +1,6 @@
 import {
   SingletonPromise
-} from "../../source/PromiseTypes.mjs";
+} from "../../source/PromiseTypes.js";
 
 describe("PromiseTypes.SingletonPromise", () => {
   it("resolves to the value we pass in", async () => {
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.ts
similarity index 99%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.ts
index ebe57fae..072b4bba 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/PropertyKeySorter.ts
@@ -1,4 +1,4 @@
-import PropertyKeySorter, { propertyKey } from "../source/PropertyKeySorter.mjs";
+import PropertyKeySorter, { propertyKey } from "../source/PropertyKeySorter.js";
 
 describe("Property key sorter", () => {
   const str0 = "0", str1 = "1", sym0 = Symbol("0"), sym1 = Symbol("1");
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.ts
similarity index 99%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.ts
index 069fccf1..a695a88e 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/ReplaceableValueMap.ts
@@ -1,4 +1,4 @@
-import ReplaceableValueMap from "#stage_utilities/source/ReplaceableValueMap.mjs";
+import ReplaceableValueMap from "#stage_utilities/source/ReplaceableValueMap.js";
 
 it("ReplaceableValueMap provides a matching replacement for each value it receives", () => {
   class Foo {}
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.ts
similarity index 98%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.ts
index cd193bd9..3511a14b 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/RequiredInitializers.ts
@@ -1,6 +1,6 @@
 import getRequiredInitializers, {
   type RequiredState
-} from "../source/RequiredInitializers.mjs";
+} from "../source/RequiredInitializers.js";
 
 it("RequiredInitializers provide a one-way path to ensuring flags are set, then cleared, then empty", () => {
   function message(
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.ts
similarity index 95%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.ts
index 924e0f00..b5f55e5d 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/_types/ClassMethodDecoratorFunction.ts
@@ -1,11 +1,11 @@
 import type {
   ClassMethodDecoratorFunction
-} from "../../source/types/ClassMethodDecoratorFunction.mjs";
+} from "../../source/types/ClassMethodDecoratorFunction.js";
 
-import NumberStringClass from "#stage_utilities/fixtures/NumberStringClass.mjs";
+import NumberStringClass from "#stage_utilities/fixtures/NumberStringClass.js";
 import type {
   NumberStringType
-} from "#stage_utilities/fixtures/types/NumberStringType.mjs";
+} from "#stage_utilities/fixtures/types/NumberStringType.js";
 
 describe("ClassMethodDecoratorFunction is compatible with ECMAScript decorators", () => {
   it("returning void", () => {
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/collections/WeakStrongMap.ts b/packages/v2-es-membrane/_01_stage_utilities/spec/collections/WeakStrongMap.ts
index dd6bfef1..e4ff5b0e 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/collections/WeakStrongMap.ts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/collections/WeakStrongMap.ts
@@ -1,6 +1,6 @@
 import WeakStrongMap from "#stage_utilities/source/collections/WeakStrongMap.js";
 
-describe("CodeGenerator(WeakStrongMap.mjs),", () => {
+describe("CodeGenerator(WeakStrongMap.js),", () => {
   let testMap: WeakStrongMap<object, unknown, unknown>, refMap = new Map;
 
   const defaultValue1 = Symbol("default value one");
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/OneToOneStrongMap.ts b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/OneToOneStrongMap.ts
index 7cfc7e12..d95fb717 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/OneToOneStrongMap.ts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/OneToOneStrongMap.ts
@@ -1,7 +1,7 @@
 import OneToOneStrongMap from "#stage_utilities/source/collections/OneToOneStrongMap.js";
-import holdsArgument from "#stage_utilities/source/gc/holdsArgument.mjs";
+import holdsArgument from "#stage_utilities/source/gc/holdsArgument.js";
 
-describe("CodeGenerator(OneToOneStrongMap.mjs) to hold values", () => {
+describe("CodeGenerator(OneToOneStrongMap.js) to hold values", () => {
   let map: OneToOneStrongMap<unknown, object>;
   beforeEach(() => map = new OneToOneStrongMap);
 
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakRefSet.ts b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakRefSet.ts
index 2a898d53..828bbc20 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakRefSet.ts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakRefSet.ts
@@ -1,5 +1,5 @@
 import WeakRefSet from "#stage_utilities/source/collections/WeakRefSet.js";
-import holdsReturn from "#stage_utilities/source/gc/holdsReturn.mjs";
+import holdsReturn from "#stage_utilities/source/gc/holdsReturn.js";
 
 it("WeakRefSet holds references to objects weakly", async () => {
   const refSet: WeakRefSet<object> = new WeakRefSet;
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakStrongMap.ts b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakStrongMap.ts
index ec81b297..6132b839 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakStrongMap.ts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/collections/WeakStrongMap.ts
@@ -1,7 +1,7 @@
 import WeakStrongMap from "#stage_utilities/source/collections/WeakStrongMap.js";
-import holdsArgument from "#stage_utilities/source/gc/holdsArgument.mjs";
+import holdsArgument from "#stage_utilities/source/gc/holdsArgument.js";
 
-describe("CodeGenerator(WeakStrongMap.mjs) holds references to objects", () => {
+describe("CodeGenerator(WeakStrongMap.js) holds references to objects", () => {
   let testMap: WeakStrongMap<object, unknown, unknown>, refMap = new Map;
 
   const defaultValue1 = Symbol("default value one");
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.ts
similarity index 99%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.ts
index 21ddf7ff..fde124a9 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsArgument.ts
@@ -1,4 +1,4 @@
-import holdsArgument from "#stage_utilities/source/gc/holdsArgument.mjs";
+import holdsArgument from "#stage_utilities/source/gc/holdsArgument.js";
 
 it("holdsArgument demonstrates functions holding an object strongly or weakly", async () => {
   function voidObject(obj: object): void {
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.ts
similarity index 98%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.ts
index 7786ff5a..a03912c9 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/garbage-collection/holdsReturn.ts
@@ -1,4 +1,4 @@
-import holdsReturn from "#stage_utilities/source/gc/holdsReturn.mjs";
+import holdsReturn from "#stage_utilities/source/gc/holdsReturn.js";
 
 it("holdsReturn demonstrates functions returning objects held strongly or weakly", async () => {
   function voidObject(): object {
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.ts
similarity index 68%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.ts
index 339a6d87..a3e9dfc5 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/getTS_SourceFile.ts
@@ -1,5 +1,5 @@
-import type { ModuleSourceDirectory } from "../source/AsyncSpecModules.mjs";
-import getTS_SourceFile from "../source/getTS_SourceFile.mjs";
+import type { ModuleSourceDirectory } from "../source/AsyncSpecModules.js";
+import getTS_SourceFile from "../source/getTS_SourceFile.js";
 
 it("getTS_SourceFile works", () => {
   const startDir: ModuleSourceDirectory = {
@@ -7,7 +7,7 @@ it("getTS_SourceFile works", () => {
     pathToDirectory: "../../source"
   };
 
-  const sourceFile = getTS_SourceFile(startDir, "PromiseTypes.mts");
+  const sourceFile = getTS_SourceFile(startDir, "PromiseTypes.ts");
   expect(
     () => sourceFile.getFunctionOrThrow("PromiseAllParallel")
   ).not.toThrow();
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.mts b/packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.ts
similarity index 99%
rename from packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.mts
rename to packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.ts
index 4af1f5ab..f27aa9ee 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.mts
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/stateMachines/dfa-states.ts
@@ -1,4 +1,4 @@
-import StateMachine_DFA from "../../source/stateMachines/dfa-states.mjs";
+import StateMachine_DFA from "../../source/stateMachines/dfa-states.js";
 
 describe("StateMachine_DFA", () => {
   type State = "one" | "two" | "three";
diff --git a/packages/v2-es-membrane/_01_stage_utilities/spec/support/jasmine.json b/packages/v2-es-membrane/_01_stage_utilities/spec/support/jasmine.json
index 410962b0..32b5a0dc 100644
--- a/packages/v2-es-membrane/_01_stage_utilities/spec/support/jasmine.json
+++ b/packages/v2-es-membrane/_01_stage_utilities/spec/support/jasmine.json
@@ -1,6 +1,5 @@
 {
   "spec_files": [
-    "./spec/**/*.mts",
     "./spec/**/*.ts"
   ],
   "stopSpecOnExpectationFailure": false
diff --git a/packages/v2-es-membrane/_02_code_generation/source/ConvertingHead.ts b/packages/v2-es-membrane/_02_code_generation/source/ConvertingHead.ts
index 86af5ddc..5661c49b 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/ConvertingHead.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/ConvertingHead.ts
@@ -45,7 +45,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphHandlerIfc.ts b/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphHandlerIfc.ts
index b7dbafd5..0b07400b 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphHandlerIfc.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphHandlerIfc.ts
@@ -19,7 +19,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphTailHandler.ts b/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphTailHandler.ts
index b9eff988..02c965d1 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphTailHandler.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/ObjectGraphTailHandler.ts
@@ -34,7 +34,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_02_code_generation/source/forwardToReflect.ts b/packages/v2-es-membrane/_02_code_generation/source/forwardToReflect.ts
index 640d7a08..d2961e0f 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/forwardToReflect.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/forwardToReflect.ts
@@ -17,7 +17,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromLocalFile.ts b/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromLocalFile.ts
index b31c188e..4b4ec6b7 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromLocalFile.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromLocalFile.ts
@@ -4,7 +4,7 @@ import {
 
 import getTS_SourceFile, {
   addSeveralSourceFiles
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   InterfaceDeclarationImpl,
diff --git a/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromTypeScriptLib.ts b/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromTypeScriptLib.ts
index 65030df5..6dd6ec59 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromTypeScriptLib.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/getInterfaces/fromTypeScriptLib.ts
@@ -10,11 +10,11 @@ import {
 
 import {
   monorepoDir
-} from "#stage_utilities/source/AsyncSpecModules.mjs";
+} from "#stage_utilities/source/AsyncSpecModules.js";
 
 import {
   addSeveralSourceFiles
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   InterfaceDeclarationImpl,
diff --git a/packages/v2-es-membrane/_02_code_generation/source/mirroring/revokedInFlight.ts b/packages/v2-es-membrane/_02_code_generation/source/mirroring/revokedInFlight.ts
index bbf927d1..ab16c260 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/mirroring/revokedInFlight.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/mirroring/revokedInFlight.ts
@@ -20,7 +20,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_02_code_generation/source/mirroring/wrapReturnValues.ts b/packages/v2-es-membrane/_02_code_generation/source/mirroring/wrapReturnValues.ts
index f857cb68..ff970f47 100644
--- a/packages/v2-es-membrane/_02_code_generation/source/mirroring/wrapReturnValues.ts
+++ b/packages/v2-es-membrane/_02_code_generation/source/mirroring/wrapReturnValues.ts
@@ -19,7 +19,7 @@ import {
 
 import {
   createSourceFileFromStructure,
-} from "#stage_utilities/source/getTS_SourceFile.mjs";
+} from "#stage_utilities/source/getTS_SourceFile.js";
 
 import {
   stageDir
diff --git a/packages/v2-es-membrane/_03_objectgraph_handlers/build/constants.ts b/packages/v2-es-membrane/_03_objectgraph_handlers/build/constants.ts
index bf1e1f99..c9009f91 100644
--- a/packages/v2-es-membrane/_03_objectgraph_handlers/build/constants.ts
+++ b/packages/v2-es-membrane/_03_objectgraph_handlers/build/constants.ts
@@ -1,7 +1,7 @@
 import {
   type ModuleSourceDirectory,
   pathToModule,
-} from "#stage_utilities/source/AsyncSpecModules.mjs";
+} from "#stage_utilities/source/AsyncSpecModules.js";
 
 const stageDir: ModuleSourceDirectory = {
   importMeta: import.meta,
diff --git a/packages/v2-es-membrane/_03_objectgraph_handlers/build/copyGenerated.ts b/packages/v2-es-membrane/_03_objectgraph_handlers/build/copyGenerated.ts
index c002c204..14791e98 100644
--- a/packages/v2-es-membrane/_03_objectgraph_handlers/build/copyGenerated.ts
+++ b/packages/v2-es-membrane/_03_objectgraph_handlers/build/copyGenerated.ts
@@ -3,7 +3,7 @@ import path from "path";
 
 import {
   projectDir,
-} from "#stage_utilities/source/AsyncSpecModules.mjs";
+} from "#stage_utilities/source/AsyncSpecModules.js";
 
 import {
   sourceGeneratedDir,
diff --git a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/RevokerManagement.ts b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/RevokerManagement.ts
index 9fa58b1b..79bcdff1 100644
--- a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/RevokerManagement.ts
+++ b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/RevokerManagement.ts
@@ -2,7 +2,7 @@ import {
   DefaultMap
 } from "#stage_utilities/source/collections/DefaultMap.js";
 import RevokerManagement from "#objectgraph_handlers/source/RevokerManagement.js";
-import holdsArgument from "#stage_utilities/source/gc/holdsArgument.mjs";
+import holdsArgument from "#stage_utilities/source/gc/holdsArgument.js";
 
 it("RevokerManagement class does not execute revokers until we revoke an object graph", () => {
   const manager: RevokerManagement = new RevokerManagement("red");
diff --git a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/garbage-collection/RevokerManagement.ts b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/garbage-collection/RevokerManagement.ts
index 5ffc0a97..16ac22c2 100644
--- a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/garbage-collection/RevokerManagement.ts
+++ b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/garbage-collection/RevokerManagement.ts
@@ -1,5 +1,5 @@
 import RevokerManagement from "#objectgraph_handlers/source/RevokerManagement.js";
-import holdsArgument from "#stage_utilities/source/gc/holdsArgument.mjs";
+import holdsArgument from "#stage_utilities/source/gc/holdsArgument.js";
 
 describe("RevokerManagement class holds references", () => {
   let manager: RevokerManagement;
diff --git a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/support/SpyProxyHandler.ts b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/support/SpyProxyHandler.ts
index d814a71f..fc7ed6e1 100644
--- a/packages/v2-es-membrane/_03_objectgraph_handlers/spec/support/SpyProxyHandler.ts
+++ b/packages/v2-es-membrane/_03_objectgraph_handlers/spec/support/SpyProxyHandler.ts
@@ -1,4 +1,4 @@
-import SpyBase from "#stage_utilities/source/SpyBase.mjs";
+import SpyBase from "#stage_utilities/source/SpyBase.js";
 
 import type {
   RequiredProxyHandler,
diff --git a/packages/v2-es-membrane/build-utilities/source/runModule.ts b/packages/v2-es-membrane/build-utilities/source/runModule.ts
index 38d45741..33b76ca7 100644
--- a/packages/v2-es-membrane/build-utilities/source/runModule.ts
+++ b/packages/v2-es-membrane/build-utilities/source/runModule.ts
@@ -41,7 +41,7 @@ export function runModule(
   const moduleHookImports = [
     //"./build-utilities/loader-hooks/debug/registration.js?hookName=one",
     "./register-hooks.js",
-    //"./node_modules/tsimp/dist/esm/hooks/import.mjs",
+    //"./node_modules/tsimp/dist/esm/hooks/import.js",
     //"./build-utilities/loader-hooks/debug/registration.js?hookName=two",
     //"./build-utilities/loader-hooks/subpath/registration.js",
     //"./build-utilities/loader-hooks/debug/registration.js?hookName=three",
diff --git a/packages/v2-es-membrane/build-utilities/spec/BuildPromise.ts b/packages/v2-es-membrane/build-utilities/spec/BuildPromise.ts
index c66c78a6..d7ca214a 100644
--- a/packages/v2-es-membrane/build-utilities/spec/BuildPromise.ts
+++ b/packages/v2-es-membrane/build-utilities/spec/BuildPromise.ts
@@ -3,7 +3,7 @@ import {
   BuildPromiseSet
 } from "../source/BuildPromise.js";
 
-describe("BuildPromise.mts: ", () => {
+describe("BuildPromise.ts: ", () => {
   let bpSet: BuildPromiseSet;
   beforeEach(() => bpSet = new BuildPromiseSet);