diff --git a/.github/workflows/test-javascript.yml b/.github/workflows/test-javascript.yml
index 0348f9b1..d63793eb 100644
--- a/.github/workflows/test-javascript.yml
+++ b/.github/workflows/test-javascript.yml
@@ -27,7 +27,7 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-        node-version: ["20.x", "22.x", "24.x"]
+        node-version: ["22.x", "24.x", "26.x"]
         include:
           - os: windows-latest
             node-version: "22.x"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 61913df9..69eaa5d3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
+### Changed
+- [JavaScript] BREAKING CHANGE: Switch to ESM ([#173](https://github.com/cucumber/query/pull/173))
 
 ## [15.0.1] - 2026-01-22
 ### Fixed
diff --git a/javascript/.mocharc.json b/javascript/.mocharc.json
index dc536371..484e010f 100644
--- a/javascript/.mocharc.json
+++ b/javascript/.mocharc.json
@@ -1,5 +1,5 @@
 {
-  "require": ["ts-node/register"],
+  "node-option": ["loader=ts-node/esm"],
   "extension": ["ts"],
   "recursive": true,
   "spec": ["src/**/*.spec.*"],
diff --git a/javascript/package.json b/javascript/package.json
index 37745751..e33ec385 100644
--- a/javascript/package.json
+++ b/javascript/package.json
@@ -2,7 +2,7 @@
   "name": "@cucumber/query",
   "version": "15.0.1",
   "description": "Cucumber Query - query messages",
-  "type": "commonjs",
+  "type": "module",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
   "scripts": {
@@ -10,6 +10,7 @@
     "clean": "rm -rf dist",
     "fix": "biome check --fix --error-on-warnings",
     "lint": "biome check --error-on-warnings",
+    "postbuild": "node -e \"require('.')\"",
     "test": "mocha",
     "prepublishOnly": "tsc -p tsconfig.build.json"
   },
diff --git a/javascript/src/Query.spec.ts b/javascript/src/Query.spec.ts
index 86604b66..d5bbdcbd 100644
--- a/javascript/src/Query.spec.ts
+++ b/javascript/src/Query.spec.ts
@@ -4,8 +4,8 @@ import path from 'node:path'
 
 import type { Envelope, TestCaseStarted } from '@cucumber/messages'
 
-import type { Lineage } from './Lineage'
-import Query from './Query'
+import type { Lineage } from './Lineage.js'
+import Query from './Query.js'
 
 describe('Query', () => {
   let cucumberQuery: Query
@@ -91,7 +91,7 @@ describe('Query', () => {
   describe('#findLineageBy', () => {
     it('returns correct lineage for a minimal scenario', async () => {
       const envelopes: ReadonlyArray<Envelope> = (
-        await fs.readFile(path.join(__dirname, '../../testdata/src/minimal.ndjson'), {
+        await fs.readFile(path.join(import.meta.dirname, '../../testdata/src/minimal.ndjson'), {
           encoding: 'utf-8',
         })
       )
@@ -116,9 +116,12 @@ describe('Query', () => {
 
     it('returns correct lineage for a pickle from an examples table', async () => {
       const envelopes: ReadonlyArray<Envelope> = (
-        await fs.readFile(path.join(__dirname, '../../testdata/src/examples-tables.ndjson'), {
-          encoding: 'utf-8',
-        })
+        await fs.readFile(
+          path.join(import.meta.dirname, '../../testdata/src/examples-tables.ndjson'),
+          {
+            encoding: 'utf-8',
+          }
+        )
       )
         .split('\n')
         .filter((line) => !!line)
@@ -147,9 +150,12 @@ describe('Query', () => {
 
     it('returns correct lineage for a pickle with background-derived steps', async () => {
       const envelopes: ReadonlyArray<Envelope> = (
-        await fs.readFile(path.join(__dirname, '../../testdata/src/rules-backgrounds.ndjson'), {
-          encoding: 'utf-8',
-        })
+        await fs.readFile(
+          path.join(import.meta.dirname, '../../testdata/src/rules-backgrounds.ndjson'),
+          {
+            encoding: 'utf-8',
+          }
+        )
       )
         .split('\n')
         .filter((line) => !!line)
diff --git a/javascript/src/Query.ts b/javascript/src/Query.ts
index 3f08c088..2cfb38e2 100644
--- a/javascript/src/Query.ts
+++ b/javascript/src/Query.ts
@@ -32,8 +32,8 @@ import {
 import { ArrayMultimap } from '@teppeis/multimaps'
 import sortBy from 'lodash.sortby'
 
-import { assert, statusOrdinal } from './helpers'
-import type { Lineage } from './Lineage'
+import { assert, statusOrdinal } from './helpers.js'
+import type { Lineage } from './Lineage.js'
 
 export default class Query {
   private meta: Meta
diff --git a/javascript/src/acceptance.spec.ts b/javascript/src/acceptance.spec.ts
index 93421afb..20738fd4 100644
--- a/javascript/src/acceptance.spec.ts
+++ b/javascript/src/acceptance.spec.ts
@@ -7,7 +7,7 @@ import { pipeline } from 'node:stream/promises'
 import { NdjsonToMessageStream } from '@cucumber/message-streams'
 import type { Envelope, Pickle } from '@cucumber/messages'
 
-import Query from './Query'
+import Query from './Query.js'
 
 const reversePickleComparator = (a: Pickle, b: Pickle): number => {
   if (a.uri !== b.uri) {
@@ -21,15 +21,15 @@ const reversePickleComparator = (a: Pickle, b: Pickle): number => {
 
 describe('Acceptance Tests', async () => {
   const sources = [
-    path.join(__dirname, '../../testdata/src/attachments.ndjson'),
-    path.join(__dirname, '../../testdata/src/empty.ndjson'),
-    path.join(__dirname, '../../testdata/src/global-hooks.ndjson'),
-    path.join(__dirname, '../../testdata/src/global-hooks-attachments.ndjson'),
-    path.join(__dirname, '../../testdata/src/hooks.ndjson'),
-    path.join(__dirname, '../../testdata/src/minimal.ndjson'),
-    path.join(__dirname, '../../testdata/src/rules.ndjson'),
-    path.join(__dirname, '../../testdata/src/examples-tables.ndjson'),
-    path.join(__dirname, '../../testdata/src/unknown-parameter-type.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/attachments.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/empty.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/global-hooks.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/global-hooks-attachments.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/hooks.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/minimal.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/rules.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/examples-tables.ndjson'),
+    path.join(import.meta.dirname, '../../testdata/src/unknown-parameter-type.ndjson'),
   ]
   const queries: Queries = {
     countMostSevereTestStepResultStatus: (query: Query) =>
@@ -316,7 +316,10 @@ describe('Acceptance Tests', async () => {
 
         const expectedResults = JSON.parse(
           fs.readFileSync(
-            path.join(__dirname, `../../testdata/src/${suiteName}.${methodName}.results.json`),
+            path.join(
+              import.meta.dirname,
+              `../../testdata/src/${suiteName}.${methodName}.results.json`
+            ),
             {
               encoding: 'utf-8',
             }
diff --git a/javascript/src/index.ts b/javascript/src/index.ts
index ce3ed4c2..2d188e32 100644
--- a/javascript/src/index.ts
+++ b/javascript/src/index.ts
@@ -1,5 +1,5 @@
-import Query from './Query'
+import Query from './Query.js'
 
-export * from './Lineage'
+export * from './Lineage.js'
 
 export { Query }