Warn when the project's TypeScript is older than the bundled version

Fixes #880

Author: sindresorhus

lib/handle-ts-files.ts

@@ -1,9 +1,40 @@
 import path from 'node:path';
 import fs from 'node:fs';
+import {createRequire} from 'node:module';
 import ts from 'typescript';
 import {getTsconfig, createFilesMatcher} from 'get-tsconfig';
 import {tsconfigDefaults} from './constants.js';
 
+let hasWarnedAboutTypeScriptVersion = false;
+
+/**
+Warns once if the project's own TypeScript is an older major than the version XO bundles. Mixing TypeScript versions in one process can crash type-aware linting (notably under pnpm), because the TypeFlags enum was renumbered in TypeScript 6.
+*/
+const warnOnOutdatedProjectTypeScript = (cwd: string): void => {
+	if (hasWarnedAboutTypeScriptVersion) {
+		return;
+	}
+
+	let projectVersion: string;
+	try {
+		const require = createRequire(path.join(cwd, 'noop.js'));
+		({version: projectVersion} = require('typescript/package.json') as {version: string});
+	} catch {
+		// No project-level TypeScript resolvable; XO's bundled version is used, so there is no mismatch.
+		return;
+	}
+
+	const projectMajor = Number.parseInt(projectVersion, 10);
+	const bundledMajor = Number.parseInt(ts.version, 10);
+
+	if (projectMajor >= bundledMajor) {
+		return;
+	}
+
+	hasWarnedAboutTypeScriptVersion = true;
+	console.warn(`XO bundles TypeScript ${ts.version}, but your project has TypeScript ${projectVersion}. Mixing TypeScript versions in one process can crash type-aware linting (notably with pnpm). Upgrade your project's \`typescript\` to ${bundledMajor} or later, or pin it (for example, a pnpm \`overrides\` entry).`);
+};
+
 const createInMemoryProgram = (files: string[], cwd: string): ts.Program | undefined => {
 	if (files.length === 0) {
 		return undefined;
@@ -74,6 +105,8 @@ If no tsconfig is found, it will create an in-memory TypeScript Program for type
 @returns The unmatched files and an in-memory TypeScript Program.
 */
 export function handleTsconfig({files, cwd, cacheLocation}: {files: string[]; cwd: string; cacheLocation?: string}) {
+	warnOnOutdatedProjectTypeScript(cwd);
+
 	const unincludedFiles: string[] = [];
 	const filesMatcherCache = new Map<string, ReturnType<typeof createFilesMatcher>>();
 

test/cli.test.ts

@@ -2,7 +2,7 @@
 import process from 'node:process';
 import fs from 'node:fs/promises';
 import path from 'node:path';
-import {availableParallelism} from 'node:os';
+import {availableParallelism, tmpdir} from 'node:os';
 import test, {describe, type TestContext} from 'node:test';
 import assert from 'node:assert/strict';
 import dedent from 'dedent';
@@ -52,6 +52,25 @@ describe('xo CLI', {concurrency: availableParallelism()}, () => {
 		assert.ok(stdout.includes(ignoredFileWarningMessage));
 	});
 
+	test('xo warns when the project TypeScript is older than the bundled version', async t => {
+		const cwd = await fs.mkdtemp(path.join(tmpdir(), 'xo-ts-version-'));
+		t.after(async () => {
+			await fs.rm(cwd, {recursive: true, force: true});
+		});
+
+		await fs.writeFile(path.join(cwd, 'package.json'), JSON.stringify({type: 'module', name: 'ts-version-test'}), 'utf8');
+		await fs.writeFile(path.join(cwd, 'tsconfig.json'), '{}', 'utf8');
+		await fs.writeFile(path.join(cwd, 'foo.ts'), dedent`export const x = 1;\n`, 'utf8');
+
+		// Simulate an older-major project-level TypeScript so the version-mismatch warning triggers.
+		await fs.mkdir(path.join(cwd, 'node_modules', 'typescript'), {recursive: true});
+		await fs.writeFile(path.join(cwd, 'node_modules', 'typescript', 'package.json'), JSON.stringify({name: 'typescript', version: '5.0.0'}), 'utf8');
+
+		const {stderr} = await $({reject: false})`node ./dist/cli --cwd ${cwd} foo.ts`;
+		assert.ok(stderr.includes('XO bundles TypeScript'));
+		assert.ok(stderr.includes('5.0.0'));
+	});
+
 	test('xo fails with exit code 2 for missing custom suppressions file', async t => {
 		const cwd = await createProject(t);
 		const filePath = path.join(cwd, 'test.js');