Preloads a webpage through an iframe.

preload.js

//Set window.load and window.show so that the child iframe element can access those functions.
window.load = load;
window.show = show;
//Convert all links to preload on hover.
[...document.querySelectorAll("a")].forEach((a) => {
    a.addEventListener("click", (e) => {
        e.preventDefault();
        show(a.href);
    });
    a.addEventListener("mouseenter", (e) => {
        //you could alter this function so that it would preload if the mouse is close, or if the user has hovered for 200ms or more, but just onmouseenter is enough for this example.
        load(a.href);
    });
});
//Preloads a webpage by creating an invisible iframe, then showing it and using history.pushState to change the URL. Only works with relative, internal URLs.
function load(page) {
    //Return a promise fulfilled once the iframe loads, so that we can await load("page") in show("url") if the page isn't loaded already.
    return new Promise((res) => {
        //If page is external don't load it, history.pushState only works with internal URLs.
        if (!isLocal(page)) return;
        //Don't load it twice.
        if (document.getElementById(id(page))) return;
        //Create an iframe
        var iframe = document.createElement("iframe");
        iframe.src = page;
        //Add it to the document
        document.documentElement.appendChild(iframe);
        //Set a unique ID.
        iframe.id = id(page);
        //And add the preload class so it's easy to remove all iframes.
        iframe.classList.add("preload");
        iframe.onload = res; //Return promise once loaded
        //Make it fill the window, but with display: none
        addStyles(iframe, {
            background: "transparent",
            width: "100vw",
            height: "100vh",
            padding: "0",
            margin: "0",
            display: "none",
            border: "none",
        });
    });
}

function addStyles(el, styles) {
    //Just to make life easier
    for (let key in styles) {
        el.style[key] = styles[key];
    }
}
async function show(page) {
    if (!isLocal(page)) {
        //Actually navigate to the page if it's not local, e.g. show('https://google.com') would just go to google.com
        window.location.href = page;
        //Stop the rest of the function.
        return;
    }
    //If the page isn't already loaded then load it.
    if (!document.getElementById(id(page))) {
        await load(page);
    }
    //Update URL
    //This step is the reason why this won't work with external URLs.
    history.pushState({}, page, page);
    //If there's current information on the page remove it.
    //This translates into "if (document.body) document.body.remove()" but it's nicer.
    document.body && document.body.remove();
    let ifr = document.getElementById(id(page));
    //Show the iframe.
    ifr.style.display = "block";
    //Get the content document of the iframe
    let doc = ifr.contentDocument;
    //For each element in the iframe make it prevent default on click, and preload on hover. 
    Array.from(doc.querySelectorAll("a"))
        .forEach((a) => {
            a.addEventListener("click", (e) => {
                //Prevent default navigation.
                e.preventDefault();
                //Call the parent window (main window's) show function and remove the current iframe
                window.parent.show(a.href);
                ifr.remove();
            });
            a.addEventListener("mouseenter", (e) => {
                //Preload the page.
                window.parent.load(a.href);
            });
        });
}

function id(page) {
    //This function gets a unique id from a page that works with CSS id standards.
    //E.g. "#/page" is not a valid ID.
    return `preload_${hash(page).toString(16)}`;
}

function hash(str, seed = 0) {
    // I could've done without the hash function, but hash functions are cool. So why not.
    let h1 = 0xdeadbeef ^ seed,
        h2 = 0x41c6ce57 ^ seed;
    for (let i = 0, ch; i < str.length; i++) {
        ch = str.charCodeAt(i);
        h1 = Math.imul(h1 ^ ch, 2654435761);
        h2 = Math.imul(h2 ^ ch, 1597334677);
    }
    h1 = Math.imul(h1 ^ (h1 >>> 16), 2246822507) ^ Math.imul(h2 ^
        (h2 >>> 13), 3266489909);
    h2 = Math.imul(h2 ^ (h2 >>> 16), 2246822507) ^ Math.imul(h1 ^
        (h1 >>> 13), 3266489909);
    return 4294967296 * (2097151 & h2) + (h1 >>> 0);
}

function isLocal(page) {
    //Tests if a page is local
    try {
        //If the page doesn't start with "http" or "https" then it's definitely local.
        if (!(page.startsWith("http://") || page.startsWith(
                "https://"))) return true;
        //If it does start with "http" or "https" we need to see if the hostname (domain) is the same as the one of the current page, so we compare the window.location.hostname with that parsed from the input URL.
        return new URL(page)
            .hostname === window.location.hostname;
    } catch (e) {
        //Invalid url maybe?
        // mailto:someone@somewhere.com will cause an error above.
        return false;
    }
}

how to use

Like this:

_$.each(document.querySelectorAll("a"), (a) => {
   a.addEventListener("click", (e) => {
       e.preventDefault();
       show(a.href);
   })
   a.addEventListener("mouseenter", (e) => {
       load(a.href);
   })
})

(untested)

Tested, it works.