sapper/templates/src/client/prefetch/index.ts

import { select_route, prefetching, set_prefetching, prepare_page } from '../app';
import { Target } from '../types';

export default function prefetch(href: string) {
	const target: Target = select_route(new URL(href, document.baseURI));

	if (target) {
		if (!prefetching || href !== prefetching.href) {
			set_prefetching(href, prepare_page(target));
		}

		return prefetching.promise;
	}
}