feat(register): resolve .js to .ts in ts-node/register

src/cjs-resolve-filename-hook.ts

@@ -1,5 +1,6 @@
 import type Module = require('module');
 import type { Service } from '.';
+import { isRelativeSpecifier } from './util';
 
 /** @internal */
 export type ModuleConstructorWithInternals = typeof Module & {
@@ -19,6 +20,10 @@ interface ModuleResolveFilenameOptions {
 
 /**
  * @internal
+ *
+ * If any features of this service require patching Module._resolveFilename,
+ * then install our hook.  Logic within the hook conditionally implements
+ * multiple resolver behaviors.
  */
 export function installCommonjsResolveHookIfNecessary(tsNodeService: Service) {
   const Module = require('module') as ModuleConstructorWithInternals;
@@ -35,7 +40,7 @@ export function installCommonjsResolveHookIfNecessary(tsNodeService: Service) {
     options?: ModuleResolveFilenameOptions,
     ...rest: any[]
   ): string {
-    if (!tsNodeService.enabled())
+    function defer(this: any) {
       return originalResolveFilename.call(
         this,
         request,
@@ -44,16 +49,60 @@ export function installCommonjsResolveHookIfNecessary(tsNodeService: Service) {
         options,
         ...rest
       );
+    }
+    if (!tsNodeService.enabled()) return defer();
 
+    // Map from emit to source extensions
+    if (
+      !isMain &&
+      canReplaceJsWithTsExt(tsNodeService, request, parent?.filename)
+    ) {
+      try {
+        return originalResolveFilename.call(
+          this,
+          request.slice(0, -3),
+          parent,
+          isMain,
+          options,
+          ...rest
+        );
+      } catch (e) {
+        const mainFile = defer();
+        if (mainFile.endsWith('.js')) {
+          //re-resolve with configured extension preference
+          return originalResolveFilename.call(
+            this,
+            mainFile.slice(0, -3),
+            parent,
+            isMain,
+            options,
+            ...rest
+          );
+        }
+        return mainFile;
+      }
+    }
     // This is a stub to support other pull requests that will be merged in the near future
     // Right now, it does nothing.
-    return originalResolveFilename.call(
-      this,
-      request,
-      parent,
-      isMain,
-      options,
-      ...rest
-    );
+    return defer();
+  }
+}
+
+function canReplaceJsWithTsExt(
+  service: Service,
+  request: string,
+  parentPath?: string
+) {
+  if (!parentPath || service.ignored(parentPath)) return false;
+  if (isRelativeSpecifier(request) && request.slice(-3) === '.js') {
+    if (!parentPath) return true;
+    const paths = require.main?.paths || [];
+    // This logic is intending to exclude node_modules
+    for (let i = 0; i < paths.length; i++) {
+      if (parentPath.startsWith(paths[i])) {
+        return false;
+      }
+    }
+    return true;
   }
 }

src/index.ts

@@ -383,6 +383,13 @@ export interface CreateOptions {
    * @default false
    */
   preferTsExts?: boolean;
+
+  /**
+   * Enable experimental resolver features, explained in full here:
+   * TODO link to an issue? or a docs page?
+   *
+   */
+  experimentalResolverFeatures?: boolean;
 }
 
 export type ModuleTypes = Record<string, 'cjs' | 'esm' | 'package'>;

src/util.ts

@@ -156,3 +156,13 @@ export function getBasePathForProjectLocalDependencyResolution(
   // and we attempt to resolve relative specifiers.  By the time we resolve relative specifiers,
   // should have configFilePath, so not reach this codepath.
 }
+
+export function isRelativeSpecifier(specifier: string) {
+  if (specifier[0] === '.') {
+    if (specifier.length === 1 || specifier[1] === '/') return true;
+    if (specifier[1] === '.') {
+      if (specifier.length === 2 || specifier[2] === '/') return true;
+    }
+  }
+  return false;
+}