JavaScript: Resource Hinting

resource-hints.js

(function() {
    let isOpera = (!!window.opr && !!opr.addons) || !!window.opera || navigator.userAgent.indexOf(' OPR/') >= 0,
        // Chrome 1+
        isChrome = !!window.chrome && !!window.chrome.webstore;

	console.log("isChrome: ", isChrome);
	console.log("isOpera: ", isOpera);
    function linkPreload(as, url) {
        let l = document.createElement("link");
        l.rel = "preload";
        l.as = as;
        l.crossOrigin = "";
        l.href = url;
        document.head.appendChild(l);
    }

    function linkPrerender(url) {
        let l = document.createElement("link");
        l.rel = "prerender";
        l.href = url;
        document.head.appendChild(l);
    }

    function linkPrefetch(url) {
        let l = document.createElement("link");
        l.rel = "prefetch";
        l.as = "html";
        l.crossOrigin = "";
        l.href = url;
        document.head.appendChild(l);
    }

    function linkPreconnect(url) {
        let l = document.createElement("link");
        l.rel = "preconnect";
        l.crossOrigin = "";
        l.href = url;
        document.head.appendChild(l);
    }
    let resourceHints = {
        "preconnect": ["example.com", "sub1.example.com", "sub2.example.com", "anotherexample.com", "anotherexample.com", "fonts.gstatic.com"],
        "preload": [{
                type: "style",
                url: "//domain.com/path/to/style.css"
            },
            {
                type: "script",
                url: "//domain.com/path/to/script.js"
            },
            {
                type: "fetch",
                url: "/domain.com/path/to/a/config.json"
            },
            {
                type: "font",
                url: "//domain.com/path/to/iconfont.eot?abc123"
            },
            {
                type: "font",
                url: "//domain.com/path/to/iconfont.eot?abc123?7ipyug#iefix"
            },
            {
                type: "font",
                url: "//domain.com/path/to/iconfont.ttf?abc123"
            },
            {
                type: "font",
                url: "//domain.com/path/to/iconfont.woff?abc123"
            },
            {
                type: "font",
                url: "//domain.com/path/to/iconfont.svg?abc123#iconfont"
            }
        ],
        "prerender": [
            "//domain.com/path/to/some/page",
            "//domain.com/path/to/some/other/page"
        ]
    };
    // Preconnect
    for (var i = 0, l = resourceHints.preconnect.length; i < l; i++) {
        linkPreconnect(resourceHints.preconnect[i]);
    }

    // Preload static assets
    for (var i = 0, l = resourceHints.preload.length; i < l; i++) {
        linkPreload(resourceHints.preload[i].type, resourceHints.preload[i].url);
    }

    // Once have link pages (not privacy/tos)
    let ;
    for (var i = 0, l = resourceHints.prerender.length; i < l; i++) {
        // Use prerender if Chrome, Opera, Samsung
        if (isOpera || isChrome) {
            linkPrerender(resourceHints.prerender[i]);
        } else {
            // Otherwise use prefetch
            linkPrefetch(resourceHints.prerender[i]);
        }
    }
})();