sveltejs/kit · error · Error

You need to install TypeScript if you want to transpile Type

Error message

You need to install TypeScript if you want to transpile TypeScript files and/or generate type definitions

What it means

`try_load_ts` dynamically imports the `typescript` package and rethrows any failure as a clear error: @sveltejs/package needs TypeScript itself to transpile TS files or generate .d.ts. TypeScript is an optional peer, so it is not auto-installed with the packager.

Source

Thrown at packages/package/src/typescript.js:217

	const path = pattern.text.startsWith('!') ? pattern.text.slice(1) : pattern.text;

	if (!(path.startsWith('./') || path.startsWith('../')) || !path.endsWith('.ts')) {
		return pattern;
	}

	const rewritten = pattern.text.slice(0, -3) + '.js';
	const literal = ts.isStringLiteral(pattern)
		? ts.factory.createStringLiteral(rewritten, pattern.getText().startsWith("'"))
		: ts.factory.createNoSubstitutionTemplateLiteral(rewritten);

	return ts.setTextRange(literal, pattern);
}

async function try_load_ts() {
	try {
		return (await import('typescript')).default;
	} catch {
		throw new Error(
			'You need to install TypeScript if you want to transpile TypeScript files and/or generate type definitions'
		);
	}
}

/**
 * @param {string | undefined} tsconfig
 * @param {string} filename
 * @param {Map<string, import('typescript').CompilerOptions>} cache
 * @param {typeof import('typescript')} [ts]
 */
export async function load_tsconfig(tsconfig, filename, cache, ts) {
	if (!ts) {
		ts = await try_load_ts();
	}

	let config_filename;
	/** @type {string[]} */

View on GitHub (pinned to 03f1687fe6)

Solutions

1. Install TypeScript: `npm i -D typescript`.
2. In pnpm workspaces, declare typescript in the package's devDependencies so it links locally.
3. Verify `node_modules/typescript` exists and imports cleanly; reinstall node_modules if corrupted.

Example fix

// before
"devDependencies": { "@sveltejs/package": "^2.0.0" }

// after
"devDependencies": { "@sveltejs/package": "^2.0.0", "typescript": "^5.0.0" }

try { await import('typescript'); } catch { throw new Error('add typescript to devDependencies'); }

try {
  await run(['svelte-package']);
} catch (e) {
  if (String(e.message).includes('You need to install TypeScript')) {
    throw new Error('Run: npm i -D typescript');
  }
  throw e;
}

When it happens

Trigger: Running `svelte-package` on a project that includes `.ts`/`.svelte` files needing transpilation or `types` generation while `typescript` is absent from node_modules (dynamic `import('typescript')` rejects).

Common situations: Library repo without typescript in devDependencies; pnpm strict installs where typescript wasn't declared; broken/incompatible typescript install causing the import to fail.

Related errors

• Could not resolve peer dependency "${name}" relative to your
• ${keypath} should be a function, if specified
• Could not resolve peer dependency "${name}" relative to your
• Could not find valid "${subpackage}" export in ${name}/packa
• Failed to locate provided tsconfig or jsconfig

AI-assisted analysis of sveltejs/kit@03f1687fe6 (2026-09-02). Data as JSON: /api/errors/0ce3a5ef2707c828. Report an issue: GitHub.