sapper/runtime/src/app/prefetch/index.ts

import { select_target, prefetching, set_prefetching, hydrate_target } from '../app';
import { Target } from '../types';

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

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

		return prefetching.promise;
	}
}