Add proper file extensions when importing a typescript file from a typescript file

lib/rules/file-extension-in-import.js

@@ -10,6 +10,11 @@ const visitImport = require("../util/visit-import")
 const packageNamePattern = /^(?:@[^/\\]+[/\\])?[^/\\]+$/u
 const corePackageOverridePattern =
     /^(?:assert|async_hooks|buffer|child_process|cluster|console|constants|crypto|dgram|dns|domain|events|fs|http|http2|https|inspector|module|net|os|path|perf_hooks|process|punycode|querystring|readline|repl|stream|string_decoder|sys|timers|tls|trace_events|tty|url|util|v8|vm|worker_threads|zlib)[/\\]$/u
+const typescriptFileExtensionsMapping = {
+    ".ts": ".js",
+    ".cts": ".cjs",
+    ".mts": ".mjs",
+}
 
 /**
  * Get all file extensions of the files which have the same basename.
@@ -31,6 +36,27 @@ function getExistingExtensions(filePath) {
     }
 }
 
+/**
+ * Get the file extension that should be added in an import statement,
+ * based on the given file extension of the referenced file.
+ *
+ * For example, in typescript, when referencing another typescript from a typescript file,
+ * a .js extension should be used instead of the original .ts extension of the referenced file.
+ * @param {string} referencedFileExt The original file extension of the referenced file.
+ * @param {string} referencingFileExt The original file extension of the file the contains the import statement.
+ * @returns {string} The file extension to append to the import statement.
+ */
+function getFileExtensionToAdd(referencedFileExt, referencingFileExt) {
+    if (
+        referencingFileExt in typescriptFileExtensionsMapping &&
+        referencedFileExt in typescriptFileExtensionsMapping
+    ) {
+        return typescriptFileExtensionsMapping[referencedFileExt]
+    }
+
+    return referencedFileExt
+}
+
 module.exports = {
     meta: {
         docs: {
@@ -85,16 +111,26 @@ module.exports = {
 
             // Verify.
             if (style === "always" && ext !== originalExt) {
+                const referencingFileExt = path.extname(
+                    context.getPhysicalFilename()
+                )
+                const fileExtensionToAdd = getFileExtensionToAdd(
+                    ext,
+                    referencingFileExt
+                )
                 context.report({
                     node,
                     messageId: "requireExt",
-                    data: { ext },
+                    data: { ext: fileExtensionToAdd },
                     fix(fixer) {
                         if (existingExts.length !== 1) {
                             return null
                         }
                         const index = node.range[1] - 1
-                        return fixer.insertTextBeforeRange([index, index], ext)
+                        return fixer.insertTextBeforeRange(
+                            [index, index],
+                            fileExtensionToAdd
+                        )
                     },
                 })
             } else if (style === "never" && ext === originalExt) {

tests/lib/rules/file-extension-in-import.js

@@ -69,6 +69,14 @@ new RuleTester({
             filename: fixture("test.js"),
             code: "import './c.mjs'",
         },
+        {
+            filename: fixture("test.js"),
+            code: "import './d.js'",
+        },
+        {
+            filename: fixture("test.ts"),
+            code: "import './d.js'",
+        },
         {
             filename: fixture("test.js"),
             code: "import './a.js'",