chore: Add tests to confirm the number of calls ro register

lib/loader.spec.ts

@@ -9,12 +9,13 @@ describe("TypeScriptLoader", () => {
   const fixturesPath = path.resolve(__dirname, "__fixtures__");
 
   let loader: Loader;
+  let tsNodeSpy = jest.spyOn(tsnode, "register");
 
   function readFixtureContent(file: string): string {
     return fs.readFileSync(file).toString();
   }
 
-  beforeEach(() => {
+  beforeAll(() => {
     loader = TypeScriptLoader();
   });
 
@@ -28,6 +29,14 @@ describe("TypeScriptLoader", () => {
     expect(() => loader(filePath, readFixtureContent(filePath))).toThrowError();
   });
 
+  it("should use the same instance of ts-node across multiple calls", () => {
+    const filePath = path.resolve(fixturesPath, "valid.fixture.ts");
+    loader(filePath, readFixtureContent(filePath));
+    loader(filePath, readFixtureContent(filePath));
+    loader(filePath, readFixtureContent(filePath));
+    expect(tsNodeSpy).toHaveBeenCalledTimes(1);
+  });
+
   it("should throw a TypeScriptCompileError on error", () => {
     try {
       const filePath = path.resolve(fixturesPath, "invalid.fixture.ts");

lib/loader.ts

@@ -3,14 +3,14 @@ import { register, RegisterOptions } from "ts-node";
 import { TypeScriptCompileError } from "./typescript-compile-error";
 
 export function TypeScriptLoader(options?: RegisterOptions): Loader {
-  const tsNodeInstance = register({ ...options, compilerOptions: { module: "commonjs" } })
+  const tsNodeInstance = register({
+    ...options,
+    compilerOptions: { module: "commonjs" },
+  });
   return (path: string, content: string) => {
     try {
       // cosmiconfig requires the transpiled configuration to be CJS
-      tsNodeInstance.compile(
-        content,
-        path
-      );
+      tsNodeInstance.compile(content, path);
       const result = require(path);
 
       // `default` is used when exporting using export default, some modules