diff --git a/eslint.config.mjs b/eslint.config.js
similarity index 100%
rename from eslint.config.mjs
rename to eslint.config.js
diff --git a/jest.config.js b/jest.config.js
index 7cf1c26f..f6a60fb2 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -1,7 +1,9 @@
-module.exports = {
-  preset: 'ts-jest',
+/** @type {import('ts-jest').JestConfigWithTsJest} */
+export default {
+  preset: 'ts-jest/presets/default-esm',
   clearMocks: true,
-  testEnvironment: 'node',
-  testMatch: ['**/*.test.ts'],
-  verbose: true
+  // https://kulshekhar.github.io/ts-jest/docs/guides/esm-support/
+  moduleNameMapper: {
+    '^(\\.{1,2}/.*)\\.js$': '$1',
+  },
 }
diff --git a/package.json b/package.json
index 7d033a22..60b4b450 100644
--- a/package.json
+++ b/package.json
@@ -6,6 +6,7 @@
     "build": "ncc build --source-map --license licenses.txt src/main.ts",
     "test": "jest"
   },
+  "type": "module",
   "dependencies": {
     "@actions/core": "1.10.1",
     "@actions/exec": "1.1.1",
diff --git a/src/comment.ts b/src/comment.ts
index 84e96a07..eb1ae6ba 100644
--- a/src/comment.ts
+++ b/src/comment.ts
@@ -1,5 +1,5 @@
 import * as core from '@actions/core'
-import { GitHubContext } from './github'
+import { GitHubContext } from './github.js'
 
 type Comment = {
   body: string
diff --git a/src/format.ts b/src/format.ts
index a872f581..cce78087 100644
--- a/src/format.ts
+++ b/src/format.ts
@@ -1,5 +1,5 @@
 import * as core from '@actions/core'
-import { Diff } from './diff'
+import { Diff } from './diff.js'
 
 type CommentOptions = {
   header: string
diff --git a/src/label.ts b/src/label.ts
index 38509eef..9f51214b 100644
--- a/src/label.ts
+++ b/src/label.ts
@@ -1,5 +1,5 @@
 import * as core from '@actions/core'
-import { GitHubContext } from './github'
+import { GitHubContext } from './github.js'
 
 export const addLabels = async (github: GitHubContext, labels: string[]): Promise<void> => {
   if (labels.length < 1) {
diff --git a/src/main.ts b/src/main.ts
index 7c0540f3..f6898253 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,7 +1,7 @@
 import * as core from '@actions/core'
-import { run } from './run'
-import { getGitHubContext } from './github'
-import { UpdateIfExistsType } from './comment'
+import { run } from './run.js'
+import { getGitHubContext } from './github.js'
+import { UpdateIfExistsType } from './comment.js'
 
 const main = async (): Promise<void> => {
   const outputs = await run(getGitHubContext(core.getInput('token', { required: true })), {
diff --git a/src/run.ts b/src/run.ts
index ed6e247d..f3ae9388 100644
--- a/src/run.ts
+++ b/src/run.ts
@@ -1,9 +1,9 @@
 import * as core from '@actions/core'
-import { GitHubContext } from './github'
-import { computeDiff, showColorDiff } from './diff'
-import { addLabels, removeLabels } from './label'
-import { UpdateIfExistsType, addComment } from './comment'
-import { formatComment } from './format'
+import { GitHubContext } from './github.js'
+import { computeDiff, showColorDiff } from './diff.js'
+import { addLabels, removeLabels } from './label.js'
+import { UpdateIfExistsType, addComment } from './comment.js'
+import { formatComment } from './format.js'
 
 type Inputs = {
   base: string
diff --git a/tests/diff.test.ts b/tests/diff.test.ts
index 1688c656..69302741 100644
--- a/tests/diff.test.ts
+++ b/tests/diff.test.ts
@@ -1,4 +1,4 @@
-import { computeDiff } from '../src/diff'
+import { computeDiff } from '../src/diff.js'
 
 test('diff', async () => {
   const diffs = await computeDiff(`${__dirname}/fixtures/base`, `${__dirname}/fixtures/head`)
diff --git a/tests/format.test.ts b/tests/format.test.ts
index 8925822f..b913c76e 100644
--- a/tests/format.test.ts
+++ b/tests/format.test.ts
@@ -1,4 +1,4 @@
-import { formatComment } from '../src/format'
+import { formatComment } from '../src/format.js'
 
 test('formatComment', () => {
   const comment = formatComment(
diff --git a/tsconfig.json b/tsconfig.json
index 1adf1fe2..92ebc1fd 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,6 +1,4 @@
 {
-  "extends": "@tsconfig/node20/tsconfig.json",
-  "compilerOptions": {
-    "outDir": "./lib"
-  }
+  "$schema": "https://json.schemastore.org/tsconfig",
+  "extends": "@tsconfig/node20/tsconfig.json"
 }