diff --git a/src/index.ts b/src/index.ts
index e2bca0f..051dc0c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,11 +1,9 @@
-import { extname } from 'path'
-
 import { Config } from '@jest/types'
 import { TransformOptions as JestTransformOptions, Transformer } from '@jest/transform'
-import { Format, Loader, TransformOptions, transformSync } from 'esbuild'
+import { transformSync } from 'esbuild'
 
 import { Options } from './options'
-import { getExt, loaders } from './utils'
+import { getEsbuildConfig } from './utils'
 
 const createTransformer = (options?: Options) => ({
   process(content: string, 
@@ -14,16 +12,6 @@ const createTransformer = (options?: Options) => ({
     opts?: JestTransformOptions
   ) {
     const sources = { code: content }
-    const ext = getExt(filename), extName = extname(filename).slice(1)
-
-    const enableSourcemaps = options?.sourcemap || false
-    const loader = (options?.loaders && options?.loaders[ext] 
-      ? options.loaders[ext]
-      : loaders.includes(extName) ? extName: 'text'
-    ) as Loader
-    const sourcemaps: Partial<TransformOptions> = enableSourcemaps 
-      ? { sourcemap: true, sourcesContent: false, sourcefile: filename } 
-      : {}
 
     /// this logic or code from 
     /// https://github.com/threepointone/esjest-transform/blob/main/src/index.js
@@ -40,17 +28,11 @@ const createTransformer = (options?: Options) => ({
       sources.code = source
     }
 
-    const result = transformSync(sources.code, {
-      loader,
-      format: options?.format as Format || 'cjs',
-      target: options?.target || 'es2018',
-      ...(options?.jsxFactory ? { jsxFactory: options.jsxFactory }: {}),
-      ...(options?.jsxFragment ? { jsxFragment: options.jsxFragment }: {}),
-      ...sourcemaps
-    })
+    const esbuildConfig = getEsbuildConfig(options, filename)
+    const result = transformSync(sources.code, esbuildConfig)
   
     let { map, code } = result;
-    if (enableSourcemaps) {
+    if (esbuildConfig.sourcemap) {
       map = {
         ...JSON.parse(result.map),
         sourcesContent: null,
diff --git a/src/utils.ts b/src/utils.ts
index 849e686..0d61dd9 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,14 +1,28 @@
 import path from 'path' 
+import { Format, Loader, TransformOptions } from 'esbuild'
 
-export const loaders = ["js", "jsx", "ts", "tsx", "json"]
+const loaders = ['js', 'jsx', 'ts', 'tsx', 'json']
 
-export const getExt = (str: string) => {
-  const basename = path.basename(str);
-  const firstDot = basename.indexOf('.');
-  const lastDot = basename.lastIndexOf('.');
-  const extname = path.extname(basename).replace(/(\.[a-z0-9]+).*/i, '$1');
+export const getEsbuildConfig = (options, filename) => {
+  const ext = path.extname(filename),
+    extName = path.extname(filename).slice(1)
+  const loader = (options?.loaders && options?.loaders[ext]
+    ? options.loaders[ext]
+    : loaders.includes(extName)
+    ? extName
+    : 'text') as Loader
 
-  if (firstDot === lastDot) return extname
+  const enableSourcemaps = options?.sourcemap || false
+  const sourcemaps: Partial<TransformOptions> = enableSourcemaps
+    ? { sourcemap: true, sourcesContent: false, sourcefile: filename }
+    : {}
 
-  return basename.slice(firstDot, lastDot) + extname
-}
\ No newline at end of file
+  return {
+    loader,
+    format: (options?.format as Format) || 'cjs',
+    target: options?.target || 'es2018',
+    ...(options?.jsxFactory ? { jsxFactory: options.jsxFactory } : {}),
+    ...(options?.jsxFragment ? { jsxFragment: options.jsxFragment } : {}),
+    ...sourcemaps,
+  }
+}
diff --git a/tests/utils.spec.ts b/tests/utils.spec.ts
new file mode 100644
index 0000000..c8a26d1
--- /dev/null
+++ b/tests/utils.spec.ts
@@ -0,0 +1,44 @@
+import { getEsbuildConfig } from '../src/utils';
+
+describe('getEsbuildConfig', () => {
+  test('with sourcemaps returns correct config', () => {
+    const config = getEsbuildConfig({ sourcemap: true }, 'filename.js')
+
+    expect(config).toEqual({
+      format: 'cjs',
+      loader: 'js',
+      target: 'es2018',
+      sourcefile: 'filename.js',
+      sourcemap: true,
+      sourcesContent: false,
+    })
+  })
+
+  describe('with loaders', () => {
+    test('.js loader returns correct config', () => {
+      const config = getEsbuildConfig(
+        { loaders: { '.js': 'jsx' } },
+        'filename.js'
+      )
+
+      expect(config).toEqual({
+        format: 'cjs',
+        loader: 'jsx',
+        target: 'es2018',
+      });
+    });
+
+    test('.js loader returns correct config with compound extension', () => {
+      const config = getEsbuildConfig(
+        { loaders: { '.js': 'jsx' } },
+        'filename.test.js'
+      )
+
+      expect(config).toEqual({
+        format: 'cjs',
+        loader: 'jsx',
+        target: 'es2018',
+      });
+    });
+  })
+})
\ No newline at end of file