diff --git a/.npmignore b/.npmignore
index 6a1cd2f..00a357f 100644
--- a/.npmignore
+++ b/.npmignore
@@ -11,3 +11,4 @@ test/
 .gitignore
 .mocharc.json
 .eslintrc.json
+eslint.config.cjs
diff --git a/.vscode/launch.json b/.vscode/launch.json
index d7083da..58603d3 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -11,11 +11,11 @@
 			"runtimeArgs": [
 				"--enable-source-maps",
 				"--test",
-				"./lib/umd/test/**/*.test.js"
+				"./lib/esm/test/**/*.test.js"
 			],
 			"env": {},
 			"sourceMaps": true,
-			"outFiles": ["${workspaceRoot}/lib/umd/**/*.js"],
+			"outFiles": ["${workspaceRoot}/lib/esm/**/*.js"],
 			"preLaunchTask": "npm: watch"
 		}
 	]
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3414a3f..d81a5fa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+4.0.0-next.1 2026-03-04
+=================
+- package is now ESM-only.
+- remove UMD build and package entrypoints.
+
+
 3.3.0 2022-06-24
 =================
 - `JSONVisitor.onObjectBegin` and `JSONVisitor.onArrayBegin` can now return `false` to instruct the visitor that no children should be visited.
diff --git a/README.md b/README.md
index 08aa17b..9092210 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,8 @@ JSONC is JSON with JavaScript style comments. This node module provides a scanne
 Installation
 ------------
 
+`jsonc-parser` is published as an ESM-only package.
+
 ```
 npm install --save jsonc-parser
 ```
diff --git a/build/remove-sourcemap-refs.js b/build/remove-sourcemap-refs.cjs
similarity index 100%
rename from build/remove-sourcemap-refs.js
rename to build/remove-sourcemap-refs.cjs
diff --git a/eslint.config.js b/eslint.config.cjs
similarity index 100%
rename from eslint.config.js
rename to eslint.config.cjs
diff --git a/package-lock.json b/package-lock.json
index 9a7d72d..43cd467 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "jsonc-parser",
-  "version": "3.3.1",
+  "version": "4.0.0-next.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "jsonc-parser",
-      "version": "3.3.1",
+      "version": "4.0.0-next.1",
       "license": "MIT",
       "devDependencies": {
         "@types/node": "22.x",
diff --git a/package.json b/package.json
index 4b40781..0a5eb82 100644
--- a/package.json
+++ b/package.json
@@ -1,10 +1,15 @@
 {
   "name": "jsonc-parser",
-  "version": "3.3.1",
+  "version": "4.0.0-next.1",
   "description": "Scanner and parser for JSON with comments.",
-  "main": "./lib/umd/main.js",
-  "typings": "./lib/umd/main.d.ts",
-  "module": "./lib/esm/main.js",
+  "type": "module",
+  "exports": {
+    ".": {
+      "types": "./lib/esm/main.d.ts",
+      "import": "./lib/esm/main.js"
+    }
+  },
+  "types": "./lib/esm/main.d.ts",
   "author": "Microsoft Corporation",
   "repository": {
     "type": "git",
@@ -23,13 +28,12 @@
     "typescript": "^5.8.3"
   },
   "scripts": {
-    "prepack": "npm run clean && npm run compile-esm && npm run test && npm run remove-sourcemap-refs",
+    "prepack": "npm run clean && npm run compile && npm run test && npm run remove-sourcemap-refs",
     "compile": "tsc -p ./src && npm run lint",
-    "compile-esm": "tsc -p ./src/tsconfig.esm.json",
-    "remove-sourcemap-refs": "node ./build/remove-sourcemap-refs.js",
+    "remove-sourcemap-refs": "node ./build/remove-sourcemap-refs.cjs",
     "clean": "rimraf lib",
     "watch": "tsc -w -p ./src",
-    "test": "npm run compile && node --enable-source-maps --test ./lib/umd/test/**/*.test.js",
+    "test": "npm run compile && node --enable-source-maps --test ./lib/esm/test/**/*.test.js",
     "lint": "eslint src/**/*.ts"
   }
 }
diff --git a/src/impl/edit.ts b/src/impl/edit.ts
index e810c6b..4ce554c 100644
--- a/src/impl/edit.ts
+++ b/src/impl/edit.ts
@@ -4,9 +4,9 @@
  *--------------------------------------------------------------------------------------------*/
 'use strict';
 
-import { Edit, ParseError, Node, JSONPath, Segment, ModificationOptions } from '../main';
-import { format, isEOL } from './format';
-import { parseTree, findNodeAtLocation } from './parser';
+import { Edit, ParseError, Node, JSONPath, Segment, ModificationOptions } from '../main.js';
+import { format, isEOL } from './format.js';
+import { parseTree, findNodeAtLocation } from './parser.js';
 
 export function removeProperty(text: string, path: JSONPath, options: ModificationOptions): Edit[] {
 	return setProperty(text, path, void 0, options);
diff --git a/src/impl/format.ts b/src/impl/format.ts
index 383f2fb..e9081fb 100644
--- a/src/impl/format.ts
+++ b/src/impl/format.ts
@@ -4,9 +4,9 @@
  *--------------------------------------------------------------------------------------------*/
 'use strict';
 
-import { Range, FormattingOptions, Edit, SyntaxKind, ScanError } from '../main';
-import { createScanner } from './scanner';
-import { cachedSpaces, cachedBreakLinesWithSpaces, supportedEols, SupportedEOL } from './string-intern';
+import { Range, FormattingOptions, Edit, SyntaxKind, ScanError } from '../main.js';
+import { createScanner } from './scanner.js';
+import { cachedSpaces, cachedBreakLinesWithSpaces, supportedEols, SupportedEOL } from './string-intern.js';
 
 export function format(documentText: string, range: Range | undefined, options: FormattingOptions): Edit[] {
 	let initialIndentLevel: number;
diff --git a/src/impl/parser.ts b/src/impl/parser.ts
index 36de808..13a2b3b 100644
--- a/src/impl/parser.ts
+++ b/src/impl/parser.ts
@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 'use strict';
 
-import { createScanner } from './scanner';
+import { createScanner } from './scanner.js';
 import {
 	JSONPath,
 	JSONVisitor,
@@ -17,7 +17,7 @@ import {
 	ScanError,
 	Segment,
 	SyntaxKind
-} from '../main';
+} from '../main.js';
 
 namespace ParseOptionsConfigs {
 	export const DEFAULT = {
diff --git a/src/impl/scanner.ts b/src/impl/scanner.ts
index 7c27882..daad8ff 100644
--- a/src/impl/scanner.ts
+++ b/src/impl/scanner.ts
@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 'use strict';
 
-import { ScanError, SyntaxKind, JSONScanner } from '../main';
+import { ScanError, SyntaxKind, JSONScanner } from '../main.js';
 
 /**
  * Creates a JSON scanner on the given text.
diff --git a/src/main.ts b/src/main.ts
index 5bd3566..7de1974 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -4,10 +4,10 @@
  *--------------------------------------------------------------------------------------------*/
 'use strict';
 
-import * as formatter from './impl/format';
-import * as edit from './impl/edit';
-import * as scanner from './impl/scanner';
-import * as parser from './impl/parser';
+import * as formatter from './impl/format.js';
+import * as edit from './impl/edit.js';
+import * as scanner from './impl/scanner.js';
+import * as parser from './impl/parser.js';
 
 /**
  * Creates a JSON scanner on the given text.
diff --git a/src/test/edit.test.ts b/src/test/edit.test.ts
index 1be0622..5b57669 100644
--- a/src/test/edit.test.ts
+++ b/src/test/edit.test.ts
@@ -6,18 +6,18 @@
 
 import * as assert from 'node:assert';
 import { suite, test } from 'node:test';
-import { Edit, FormattingOptions, ModificationOptions, modify } from '../main';
+import { Edit, FormattingOptions, ModificationOptions, modify } from '../main.js';
 
 suite('JSON - edits', () => {
 
 	function assertEdit(content: string, edits: Edit[], expected: string) {
-		assert(edits);
+		assert.ok(edits);
 		let lastEditOffset = content.length;
 		for (let i = edits.length - 1; i >= 0; i--) {
 			let edit = edits[i];
-			assert(edit.offset >= 0 && edit.length >= 0 && edit.offset + edit.length <= content.length);
-			assert(typeof edit.content === 'string');
-			assert(lastEditOffset >= edit.offset + edit.length); // make sure all edits are ordered
+			assert.ok(edit.offset >= 0 && edit.length >= 0 && edit.offset + edit.length <= content.length);
+			assert.ok(typeof edit.content === 'string');
+			assert.ok(lastEditOffset >= edit.offset + edit.length); // make sure all edits are ordered
 			lastEditOffset = edit.offset;
 			content = content.substring(0, edit.offset) + edit.content + content.substring(edit.offset + edit.length);
 		}
diff --git a/src/test/format.test.ts b/src/test/format.test.ts
index abd59b4..3d66fef 100644
--- a/src/test/format.test.ts
+++ b/src/test/format.test.ts
@@ -6,8 +6,8 @@
 
 import * as assert from 'node:assert';
 import { suite, test } from 'node:test';
-import * as Formatter from '../impl/format';
-import { Range } from '../main';
+import * as Formatter from '../impl/format.js';
+import { Range } from '../main.js';
 
 suite('JSON - formatter', () => {
 
diff --git a/src/test/json.test.ts b/src/test/json.test.ts
index 245b25a..f6c7601 100644
--- a/src/test/json.test.ts
+++ b/src/test/json.test.ts
@@ -9,7 +9,7 @@ import { suite, test } from 'node:test';
 import {
 	SyntaxKind, createScanner, parse, getLocation, Node, ParseError, parseTree, ParseErrorCode,
 	ParseOptions, Segment, findNodeAtLocation, getNodeValue, getNodePath, ScanError, visit, JSONVisitor, JSONPath
-} from '../main';
+} from '../main.js';
 
 function assertKinds(text: string, ...kinds: SyntaxKind[]): void {
 	var scanner = createScanner(text);
@@ -44,7 +44,7 @@ function assertInvalidParse(input: string, expected: any, options?: ParseOptions
 	var errors: ParseError[] = [];
 	var actual = parse(input, errors, options);
 
-	assert(errors.length > 0);
+	assert.ok(errors.length > 0);
 	assert.deepStrictEqual(actual, expected);
 }
 
@@ -118,7 +118,7 @@ function assertLocation(input: string, expectedSegments: Segment[], expectedNode
 	var offset = input.indexOf('|');
 	input = input.substring(0, offset) + input.substring(offset + 1, input.length);
 	var actual = getLocation(input, offset);
-	assert(actual);
+	assert.ok(actual);
 	assert.deepStrictEqual(actual.path, expectedSegments, input);
 	assert.strictEqual(actual.previousNode && actual.previousNode.type, expectedNodeType, input);
 	assert.strictEqual(actual.isAtPropertyKey, expectedCompleteProperty, input);
@@ -128,7 +128,7 @@ function assertMatchesLocation(input: string, matchingSegments: Segment[], expec
 	var offset = input.indexOf('|');
 	input = input.substring(0, offset) + input.substring(offset + 1, input.length);
 	var actual = getLocation(input, offset);
-	assert(actual);
+	assert.ok(actual);
 	assert.strictEqual(actual.matches(matchingSegments), expectedResult);
 }
 
diff --git a/src/test/string-intern.test.ts b/src/test/string-intern.test.ts
index 2e5dd6d..b1a967e 100644
--- a/src/test/string-intern.test.ts
+++ b/src/test/string-intern.test.ts
@@ -1,6 +1,6 @@
 import * as assert from 'node:assert';
 import { suite, test } from 'node:test';
-import { cachedBreakLinesWithSpaces, cachedSpaces, supportedEols } from '../impl/string-intern';
+import { cachedBreakLinesWithSpaces, cachedSpaces, supportedEols } from '../impl/string-intern.js';
 
 suite('string intern', () => {
   test('should correctly define spaces intern', () => {
diff --git a/src/tsconfig.esm.json b/src/tsconfig.esm.json
deleted file mode 100644
index 8c8246c..0000000
--- a/src/tsconfig.esm.json
+++ /dev/null
@@ -1,16 +0,0 @@
-{
-	"compilerOptions": {
-		"target": "es2020",
-		"module": "es6",
-		"moduleResolution": "node",
-		"sourceMap": true,
-		"declaration": true,
-		"stripInternal": true,
-		"outDir": "../lib/esm",
-		"strict": true,
-		"preserveConstEnums": true,
-		"lib": [
-			"es2020"
-		]
-	}
-}
\ No newline at end of file
diff --git a/src/tsconfig.json b/src/tsconfig.json
index d399fcb..ffabd33 100644
--- a/src/tsconfig.json
+++ b/src/tsconfig.json
@@ -1,12 +1,12 @@
 {
 	"compilerOptions": {
 		"target": "es2020",
-		"module": "umd",
-		"moduleResolution": "node",
+		"module": "NodeNext",
+		"moduleResolution": "NodeNext",
 		"sourceMap": true,
 		"declaration": true,
 		"stripInternal": true,
-		"outDir": "../lib/umd",
+		"outDir": "../lib/esm",
 		"strict": true,
 		"preserveConstEnums": true,
 		"lib": [