Allow opting-in to .ts import specifiers

src/configuration.ts

@@ -383,6 +383,7 @@ function filterRecognizedTsConfigTsNodeOptions(jsonObject: any): {
     experimentalResolver,
     esm,
     experimentalSpecifierResolution,
+    experimentalTsImportSpecifiers,
     ...unrecognized
   } = jsonObject as TsConfigOptions;
   const filteredTsConfigOptions = {
@@ -409,6 +410,7 @@ function filterRecognizedTsConfigTsNodeOptions(jsonObject: any): {
     experimentalResolver,
     esm,
     experimentalSpecifierResolution,
+    experimentalTsImportSpecifiers,
   };
   // Use the typechecker to make sure this implementation has the correct set of properties
   const catchExtraneousProps: keyof TsConfigOptions =

src/file-extensions.ts

@@ -19,6 +19,13 @@ const nodeEquivalents = new Map<string, string>([
   ['.cts', '.cjs'],
 ]);
 
+const tsResolverEquivalents = new Map<string, readonly string[]>([
+  ['.ts', ['.js']],
+  ['.tsx', ['.js', '.jsx']],
+  ['.mts', ['.mjs']],
+  ['.cts', ['.cjs']],
+]);
+
 // All extensions understood by vanilla node
 const vanillaNodeExtensions: readonly string[] = [
   '.js',
@@ -129,6 +136,19 @@ export function getExtensions(
      * as far as getFormat is concerned.
      */
     nodeEquivalents,
+    /**
+     * Mapping from extensions rejected by TSC in import specifiers, to the
+     * possible alternatives that TS's resolver will accept.
+     *
+     * When we allow users to opt-in to .ts extensions in import specifiers, TS's
+     * resolver requires us to replace the .ts extensions with .js alternatives.
+     * Otherwise, resolution fails.
+     *
+     * Note TS's resolver is only used by, and only required for, typechecking.
+     * This is separate from node's resolver, which we hook separately and which
+     * does not require this mapping.
+     */
+    tsResolverEquivalents,
     /**
      * Extensions that we can support if the user upgrades their typescript version.
      * Used when raising hints.

src/index.ts

@@ -373,6 +373,17 @@ export interface CreateOptions {
    * For details, see https://nodejs.org/dist/latest-v18.x/docs/api/esm.html#customizing-esm-specifier-resolution-algorithm
    */
   experimentalSpecifierResolution?: 'node' | 'explicit';
+  /**
+   * Allow using voluntary `.ts` file extension in import specifiers.
+   *
+   * Typically, in ESM projects, import specifiers must hanve an emit extension, `.js`, `.cjs`, or `.mjs`,
+   * and we automatically map to the corresponding `.ts`, `.cts`, or `.mts` source file.  This is the
+   * recommended approach.
+   *
+   * However, if you really want to use `.ts` in import specifiers, and are aware that this may
+   * break tooling, you can enable this flag.
+   */
+  experimentalTsImportSpecifiers?: boolean;
 }
 
 export type ModuleTypes = Record<string, ModuleTypeOverride>;
@@ -693,6 +704,11 @@ export function createFromPreloadedConfig(
         6059, // "'rootDir' is expected to contain all source files."
         18002, // "The 'files' list in config file is empty."
         18003, // "No inputs were found in config file."
+        ...(options.experimentalTsImportSpecifiers
+          ? [
+              2691, // "An import path cannot end with a '.ts' extension. Consider importing '<specifier without ext>' instead."
+            ]
+          : []),
         ...(options.ignoreDiagnostics || []),
       ].map(Number),
     },
@@ -905,6 +921,8 @@ export function createFromPreloadedConfig(
     patterns: options.moduleTypes,
   });
 
+  const extensions = getExtensions(config, options, ts.version);
+
   // Use full language services when the fast option is disabled.
   if (!transpileOnly) {
     const fileContents = new Map<string, string>();
@@ -985,6 +1003,8 @@ export function createFromPreloadedConfig(
         cwd,
         config,
         projectLocalResolveHelper,
+        options,
+        extensions,
       });
       serviceHost.resolveModuleNames = resolveModuleNames;
       serviceHost.getResolvedModuleWithFailedLookupLocationsFromCache =
@@ -1143,6 +1163,8 @@ export function createFromPreloadedConfig(
         ts,
         getCanonicalFileName,
         projectLocalResolveHelper,
+        options,
+        extensions,
       });
       host.resolveModuleNames = resolveModuleNames;
       host.resolveTypeReferenceDirectives = resolveTypeReferenceDirectives;
@@ -1448,7 +1470,6 @@ export function createFromPreloadedConfig(
   let active = true;
   const enabled = (enabled?: boolean) =>
     enabled === undefined ? active : (active = !!enabled);
-  const extensions = getExtensions(config, options, ts.version);
   const ignored = (fileName: string) => {
     if (!active) return true;
     const ext = extname(fileName);

src/resolver-functions.ts

@@ -1,4 +1,6 @@
 import { resolve } from 'path';
+import type { CreateOptions } from '.';
+import type { Extensions } from './file-extensions';
 import type { TSCommon, TSInternal } from './ts-compiler-types';
 import type { ProjectLocalResolveHelper } from './util';
 
@@ -13,6 +15,8 @@ export function createResolverFunctions(kwargs: {
   getCanonicalFileName: (filename: string) => string;
   config: TSCommon.ParsedCommandLine;
   projectLocalResolveHelper: ProjectLocalResolveHelper;
+  options: CreateOptions;
+  extensions: Extensions;
 }) {
   const {
     host,
@@ -21,6 +25,8 @@ export function createResolverFunctions(kwargs: {
     cwd,
     getCanonicalFileName,
     projectLocalResolveHelper,
+    options,
+    extensions,
   } = kwargs;
   const moduleResolutionCache = ts.createModuleResolutionCache(
     cwd,
@@ -105,7 +111,7 @@ export function createResolverFunctions(kwargs: {
               i
             )
           : undefined;
-        const { resolvedModule } = ts.resolveModuleName(
+        let { resolvedModule } = ts.resolveModuleName(
           moduleName,
           containingFile,
           config.options,
@@ -114,6 +120,25 @@ export function createResolverFunctions(kwargs: {
           redirectedReference,
           mode
         );
+        if (!resolvedModule && options.experimentalTsImportSpecifiers) {
+          const lastDotIndex = moduleName.lastIndexOf('.');
+          const ext = lastDotIndex >= 0 ? moduleName.slice(lastDotIndex) : '';
+          if (ext) {
+            const replacements = extensions.tsResolverEquivalents.get(ext);
+            for (const replacementExt of replacements ?? []) {
+              ({ resolvedModule } = ts.resolveModuleName(
+                moduleName.slice(0, -ext.length) + replacementExt,
+                containingFile,
+                config.options,
+                host,
+                moduleResolutionCache,
+                redirectedReference,
+                mode
+              ));
+              if (resolvedModule) break;
+            }
+          }
+        }
         if (resolvedModule) {
           fixupResolvedModule(resolvedModule);
         }

src/test/ts-import-specifiers.spec.ts

@@ -0,0 +1,22 @@
+import { context } from './testlib';
+import * as expect from 'expect';
+import { createExec } from './exec-helpers';
+import {
+  TEST_DIR,
+  ctxTsNode,
+  CMD_TS_NODE_WITHOUT_PROJECT_FLAG,
+} from './helpers';
+
+const exec = createExec({
+  cwd: TEST_DIR,
+});
+
+const test = context(ctxTsNode);
+
+test('Supports .ts extensions in import specifiers with typechecking, even though vanilla TS checker does not', async () => {
+  const { err, stdout } = await exec(
+    `${CMD_TS_NODE_WITHOUT_PROJECT_FLAG} ts-import-specifiers/index.ts`
+  );
+  expect(err).toBe(null);
+  expect(stdout.trim()).toBe('{ foo: true, bar: true }');
+});

tests/ts-import-specifiers/bar.tsx

@@ -0,0 +1 @@
+export const bar = true;

tests/ts-import-specifiers/foo.ts

@@ -0,0 +1 @@
+export const foo = true;

tests/ts-import-specifiers/index.ts

@@ -0,0 +1,3 @@
+import { foo } from './foo.ts';
+import { bar } from './bar.jsx';
+console.log({ foo, bar });

tests/ts-import-specifiers/tsconfig.json

@@ -0,0 +1,10 @@
+{
+  "ts-node": {
+    // Can eventually make this a stable feature.  For now, `experimental` flag allows me to iterate quickly
+    "experimentalTsImportSpecifiers": true,
+    "experimentalResolver": true
+  },
+  "compilerOptions": {
+    "jsx": "react"
+  }
+}