feat: importing ts files using their corresponding js extesions

packages/playground/resolve/__tests__/resolve.spec.ts

@@ -54,6 +54,10 @@ test('dont add extension to directory name (./dir-with-ext.js/index.js)', async
   expect(await page.textContent('.dir-with-ext')).toMatch('[success]')
 })
 
+test('a ts module can import another ts module using its corresponding js file name', async () => {
+  expect(await page.textContent('.ts-extension')).toMatch('[success]')
+})
+
 test('filename with dot', async () => {
   expect(await page.textContent('.dot')).toMatch('[success]')
 })

packages/playground/resolve/index.html

@@ -33,6 +33,11 @@ <h2>Resolve to non-duplicated file extension</h2>
 <h2>Don't add extensions to directory names</h2>
 <p class="dir-with-ext">fail</p>
 
+<h2>
+  A ts module can import another ts module using its corresponding js file name
+</h2>
+<p class="ts-extension">fail</p>
+
 <h2>Resolve file name containing dot</h2>
 <p class="dot">fail</p>
 
@@ -119,6 +124,9 @@ <h2>resolve package that contains # in path</h2>
   import { file as dirWithExtMsg } from './dir-with-ext'
   text('.dir-with-ext', dirWithExtMsg)
 
+  import { msg as tsExtensionMsg } from './ts-extension'
+  text('.ts-extension', tsExtensionMsg)
+
   // filename with dot
   import { bar } from './util/bar.util'
   text('.dot', bar())

packages/playground/resolve/ts-extension/hello.ts

@@ -0,0 +1 @@
+export const msg = '[success] use .js extension to import a ts module'

packages/playground/resolve/ts-extension/index.ts

@@ -0,0 +1,3 @@
+import { msg } from './hello.js'
+
+export { msg }

packages/playground/resolve/vite.config.js

@@ -2,7 +2,7 @@ const virtualFile = '@virtual-file'
 
 module.exports = {
   resolve: {
-    extensions: ['.mjs', '.js', '.es'],
+    extensions: ['.mjs', '.js', '.es', '.ts'],
     mainFields: ['custom', 'module'],
     conditions: ['custom']
   },

packages/vite/src/node/plugins/resolve.ts

@@ -25,7 +25,10 @@ import {
   cleanUrl,
   slash,
   nestedResolveFrom,
-  isFileReadable
+  isFileReadable,
+  isTsRequest,
+  isPossibleTsOutput,
+  getTsSrcPath
 } from '../utils'
 import { ViteDevServer, SSROptions } from '..'
 import { createFilter } from '@rollup/pluginutils'
@@ -65,6 +68,11 @@ export interface InternalResolveOptions extends ResolveOptions {
   skipPackageJson?: boolean
   preferRelative?: boolean
   isRequire?: boolean
+  // #3040
+  // when the importer is a ts module,
+  // if the specifier requests a non-existent `.js/jsx/mjs/cjs` file,
+  // should also try import from `.ts/tsx/mts/cts` source file as fallback.
+  isFromTsImporter?: boolean
 }
 
 export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
@@ -75,10 +83,6 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
     ssrConfig,
     preferRelative = false
   } = baseOptions
-  const requireOptions: InternalResolveOptions = {
-    ...baseOptions,
-    isRequire: true
-  }
   let server: ViteDevServer | undefined
 
   const { target: ssrTarget, noExternal: ssrNoExternal } = ssrConfig ?? {}
@@ -104,13 +108,19 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
       const targetWeb = !ssr || ssrTarget === 'webworker'
 
       // this is passed by @rollup/plugin-commonjs
-      const isRequire =
+      const isRequire = !!(
         resolveOpts &&
         resolveOpts.custom &&
         resolveOpts.custom['node-resolve'] &&
         resolveOpts.custom['node-resolve'].isRequire
+      )
 
-      const options = isRequire ? requireOptions : baseOptions
+      const options: InternalResolveOptions = {
+        ...baseOptions,
+
+        isRequire,
+        isFromTsImporter: isTsRequest(importer ?? '')
+      }
 
       const preserveSymlinks = !!server?.config.resolve.preserveSymlinks
 
@@ -450,6 +460,22 @@ function tryResolveFile(
       if (index) return index + postfix
     }
   }
+
+  const tryTsExtension = options.isFromTsImporter && isPossibleTsOutput(file)
+  if (tryTsExtension) {
+    const tsSrcPath = getTsSrcPath(file)
+    return tryResolveFile(
+      tsSrcPath,
+      postfix,
+      options,
+      tryIndex,
+      targetWeb,
+      preserveSymlinks,
+      tryPrefix,
+      skipPackageJson
+    )
+  }
+
   if (tryPrefix) {
     const prefixed = `${path.dirname(file)}/${tryPrefix}${path.basename(file)}`
     return tryResolveFile(

packages/vite/src/node/utils.ts

@@ -164,6 +164,14 @@ export const isJSRequest = (url: string): boolean => {
   return false
 }
 
+const knownTsRE = /\.(ts|mts|cts|tsx)$/
+const knownTsOutputRE = /\.(js|mjs|cjs|jsx)$/
+export const isTsRequest = (url: string) => knownTsRE.test(cleanUrl(url))
+export const isPossibleTsOutput = (url: string) =>
+  knownTsOutputRE.test(cleanUrl(url))
+export const getTsSrcPath = (filename: string) =>
+  filename.replace(/\.([cm])?(js)(x?)$/, '.$1ts$3')
+
 const importQueryRE = /(\?|&)import=?(?:&|$)/
 const internalPrefixes = [
   FS_PREFIX,