Support .ts module imports in TypeSpec source files

.chronus/changes/copilot-support-importing-typescript-files-2026-5-25-13-53-33.md

@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@typespec/compiler"
+---
+
+Support importing .ts decorator modules from TypeSpec source files.

packages/compiler/src/core/source-file.ts

@@ -40,7 +40,7 @@ export function createSourceFile(text: string, path: string): SourceFile {
 
 export function getSourceFileKindFromExt(path: string): SourceFileKind | undefined {
   const ext = getAnyExtensionFromPath(path);
-  if (ext === ".js" || ext === ".mjs") {
+  if (ext === ".js" || ext === ".mjs" || ext === ".ts") {
     return "js";
   } else if (ext === ".tsp") {
     return "typespec";

packages/compiler/test/checker/imports.test.ts

@@ -68,6 +68,29 @@ describe("compiler: imports", () => {
     expectFileLoaded(program, { typespec: ["proj/main.tsp"], js: ["blue.js"] });
   });
 
+  it("import relative TS file", async () => {
+    const { program } = await Tester.files({
+      "blue.ts": mockFile.js({ $blue() {} }),
+    }).compile(`
+      import "./blue.ts";
+      @blue
+      model A {}
+    `);
+    expectFileLoaded(program, { typespec: ["main.tsp"], js: ["blue.ts"] });
+  });
+
+  it("import relative TS file in parent folder", async () => {
+    const { program } = await Tester.files({
+      "blue.ts": mockFile.js({ $blue() {} }),
+      "proj/main.tsp": `
+      import "../blue.ts";
+      @blue
+      model A {}
+      `,
+    }).compile(`import "./proj/main.tsp";`);
+    expectFileLoaded(program, { typespec: ["proj/main.tsp"], js: ["blue.ts"] });
+  });
+
   it("import directory with main.tsp", async () => {
     const { program } = await Tester.files({
       "test/main.tsp": `model C { }`,

packages/compiler/test/e2e/scenarios/import-relative-ts/decorators.ts

@@ -0,0 +1,4 @@
+export function $blue(context: unknown, target: unknown): void {
+  void context;
+  void target;
+}

packages/compiler/test/e2e/scenarios/import-relative-ts/main.tsp

@@ -0,0 +1,4 @@
+import "./decorators.ts";
+
+@blue
+model A {}

packages/compiler/test/e2e/scenarios/scenarios.e2e.ts

@@ -130,6 +130,11 @@ describe("compiler: entrypoints", () => {
       expectDiagnosticEmpty(program.diagnostics);
     });
 
+    it("succeed with relative .ts module import", async () => {
+      const program = await compileScenario("import-relative-ts");
+      expectDiagnosticEmpty(program.diagnostics);
+    });
+
     it("succeed if loading different install of the same library at the same version", async () => {
       const program = await compileScenario("same-library-same-version", {
         emit: ["@typespec/lib2"],

website/src/content/docs/docs/language-basics/imports.md

@@ -7,7 +7,7 @@ llmstxt: true
 
 Imports are used to include files or libraries into your TypeSpec program. When compiling a TypeSpec file, you specify the path to your root TypeSpec file, typically named "main.tsp". From this root file, any imported files are added to your program. If a directory is imported, TypeSpec will search for a `main.tsp` file within that directory.
 
-The path specified in the import must either start with `"./"` or `"../"`, or be an absolute path. The path should either point to a directory, or have an extension of either ".tsp" or ".js". The examples below illustrate how to use imports to assemble a TypeSpec program from multiple files:
+The path specified in the import must either start with `"./"` or `"../"`, or be an absolute path. The path should either point to a directory, or have an extension of either ".tsp", ".js", or ".ts". The examples below illustrate how to use imports to assemble a TypeSpec program from multiple files:
 
 ## Importing a TypeSpec file