Lazy loading fonts and styles (JS)

lazy_load.js

function loadFont(name, local=[],
    eot=undefined, woff=undefined, woff2=undefined, ttf=undefined,
    font_weight="normal", font_style="normal", font_display="auto") {

    name = `font-family:'${name}';`;

    let eot_sep = (eot == undefined ? '' : `src:url('${eot}');`);

    paths = [];
    for (let path of local) paths.push(`local('${path}')`);

    if (eot   !== undefined) paths.push(`url('${eot}?#iefix')format('embedded-opentype')`);
    if (woff2 !== undefined) paths.push(`url('${woff2}')format('woff2')`);
    if (woff  !== undefined) paths.push(`url('${woff}')format('woff')`);
    if (ttf   !== undefined) paths.push(`url('${ttf}')format('truetype')`);

    paths = paths.join(",");
    paths = `src:${paths};`

    font_weight = `font-weight:${font_weight};`;
    font_style = `font-style:${font_style};`;
    font_display = `font-display:${font_display}`;  // Last ';' redundant


    let style_obj = document.createElement("style");

    style_obj.innerHTML = `@font-face{${name}${eot_sep}${paths}${font_weight}${font_style}${font_display}}`;
    document.head.appendChild(style_obj);
}


function loadStyle(url, media=undefined) {
    let style_obj = document.createElement("link");

    style_obj.rel = "stylesheet";
    style_obj.type = "text/css";
    if (media !== undefined) style_obj.media = media;
    style_obj.href = url;

    document.head.appendChild(style_obj);
}


function loadScript(url, async=false) {
    let script_obj = document.createElement("script");

    script_obj.type = "text/javascript";
    script_obj.src = url;
    script_obj.async = async;

    document.body.appendChild(script_obj);
}

function loadHTMLScripts() {
    for (let elem of document.body.getElementsByTagName("*")) {
        if (elem.tagName.toLowerCase() == "script" && elem.dataset.src !== undefined) {
            loadScript(elem.dataset.src);
            elem.remove();
        }
    }
}


// Usage:

loadFont("Segoe UI",
    local=["Segoe UI Bold", "SegoeUIBold", "SegoeUI-Bold", "Source-UI-Bold"],
    eot="/files/fonts/SegoeUI/SegoeUI-Bold.eot",
    woff="/files/fonts/SegoeUI/SegoeUI-Bold.woff",
    woff2="/files/fonts/SegoeUI/SegoeUI-Bold.woff2",
    ttf="/files/fonts/SegoeUI/SegoeUI-Bold.ttf",

    font_weight="bold", font_style="normal", font_display="swap"
);

loadStyle("build/mobile.css");

loadHTMLScripts();