next/server/api/exists/username.get.ts

import { usernameRegex } from "@server/constants";
import { User } from "@models/user";

export default eventHandler(async (event) => {
	const query = getQuery(event);
	const u = await User.findOne({
		username: usernameRegex(query.username as string),
	}).exec();
	if (u) {
		return {
			exists: true,
		};
	} else {
		return { exists: false };
	}
});