Next.js Proximity Prefetch (PPF)

proximity-prefetch.tsx

"use client";

import { useRouter } from "next/navigation";
import type { ReactNode } from "react";
import { useCallback, useEffect, useState } from "react";

interface ProximityPrefetchProps {
  children: ReactNode;
  threshold?: number;
  predictionInterval?: number;
}

export function ProximityPrefetch({
  children,
  threshold = 200,
  predictionInterval = 0,
}: ProximityPrefetchProps) {
  const router = useRouter();
  const [mousePosition, setMousePosition] = useState({ x: 0, y: 0 });
  const [prefetchedRoutes, setPrefetchedRoutes] = useState<Set<string>>(
    new Set(),
  );
  const [links, setLinks] = useState<
    { el: HTMLAnchorElement; href: string; rect: DOMRect }[]
  >([]);

  const updateLinks = useCallback(() => {
    const anchors = Array.from(
      document.querySelectorAll('a[href^="/"]'),
    ) as HTMLAnchorElement[];
    setLinks(
      anchors
        .map((el) => {
          const href = el.getAttribute("href");
          if (href?.startsWith("/") && !href.includes("#")) {
            return {
              el,
              href,
              rect: el.getBoundingClientRect(),
            };
          }
          return null;
        })
        .filter(Boolean) as {
        el: HTMLAnchorElement;
        href: string;
        rect: DOMRect;
      }[],
    );
  }, []);

  const calculateDistance = (
    x1: number,
    y1: number,
    x2: number,
    y2: number,
  ) => {
    return Math.sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2);
  };

  const calculateCenterPoint = (rect: DOMRect) => {
    return {
      x: rect.left + rect.width / 2,
      y: rect.top + rect.height / 2,
    };
  };

  const prefetchNearbyRoutes = useCallback(async () => {
    if (!links.length) return;

    // Sort links by proximity to current mouse position
    const linksWithDistance = links.map((link) => {
      const center = calculateCenterPoint(link.rect);
      const distance = calculateDistance(
        mousePosition.x,
        mousePosition.y,
        center.x,
        center.y,
      );
      return { ...link, distance };
    });

    // Sort by distance
    linksWithDistance.sort((a, b) => a.distance - b.distance);

    // Prefetch the closest links that are within threshold
    const closestLinks = linksWithDistance.filter(
      (link) => link.distance < threshold,
    );

    const routesToPrefetch = closestLinks.map((link) => link.href);

    // Prefetch up to 3 routes at a time
    for (const route of routesToPrefetch.slice(0, 3)) {
      if (!prefetchedRoutes.has(route)) {
        console.log("prefetching", route);
        router.prefetch(route);
        setPrefetchedRoutes((prev) => new Set([...prev, route]));
      }
    }
  }, [links, mousePosition, prefetchedRoutes, router, threshold]);

  useEffect(() => {
    const handleMouseMove = (e: MouseEvent) => {
      setMousePosition({ x: e.clientX, y: e.clientY });
    };

    window.addEventListener("mousemove", handleMouseMove);
    return () => {
      window.removeEventListener("mousemove", handleMouseMove);
    };
  }, []);

  useEffect(() => {
    // Update links on mount and when DOM changes
    updateLinks();

    // Set up a MutationObserver to detect new links
    const observer = new MutationObserver(() => {
      updateLinks();
    });

    observer.observe(document.body, {
      childList: true,
      subtree: true,
      attributes: true,
      attributeFilter: ["href"],
    });

    return () => {
      observer.disconnect();
    };
  }, [updateLinks]);

  useEffect(() => {
    const intervalId = setInterval(() => {
      if (mousePosition.x !== 0 || mousePosition.y !== 0) {
        prefetchNearbyRoutes();
      }
    }, predictionInterval);

    return () => {
      clearInterval(intervalId);
    };
  }, [mousePosition, prefetchNearbyRoutes, predictionInterval]);

  return children;
}

This is great ty for sharing @pontusab 🔥 🚀

Thank you.

getBoundingClientRect forces a reflow for any link that is queried on mount / when any element changes (!). I think a more performant solution would be to use IntersectionObservers, where you get the bounding client rect for basically free right after starting observing.

Also worth noting, there is also predictive prefetch (compared to proximity), where e.g. mouse acceleration is taken into account: https://github.com/callumacrae/futurelink / https://github.com/mathisonian/premonish/tree/master. Not saying either approach is strictly better, but worth mentioning.

Yeah tbh a better approach would be to add this directly to a wrapped Link component instead!

Did you see this PR ? I think it is related
vercel/next.js#77866