include(): JavaScript function for dynamically including a CSS or JS file on a page

include-callback.js

/**
 * Includes the CSS or JS file into the head of the page.
 *
 * Examples:
 *   include('http://example.com/script.js')
 *   include('http://example.com/style.css')
 *   include('http://example.com/script.php', 'js')
 * @param {string} url URL of the CSS or JS file
 * @param {string} [type] 'css' or 'js' to specify the type of file.
 *                        This parameter is optional, but it should be 
 *                        used if the url does not end in '.css' or '.js'.
 */
const include = (url, type = url.split('.').pop()).toLowerCase(), onload = function() {}, onerror = function() {}) => {
    let elem;
    
    if (type === 'css') {
        elem = document.createElement('link');
        elem.rel = 'stylesheet';
        elem.href = url;
    } else if (type === 'js') {
        elem = document.createElement('script');
        elem.src = url;
        elem.async = false;
    } else {
        throw new Error('Failed to include ' + url + ' due to unknown file type.');
    }
    
    elem.onload = onload;
    elem.onerror = onerror;
    document.head.appendChild(elem);
}

include-promise.js

/**
 * Includes the CSS or JS file into the head of the page.
 *
 * Examples:
 *   include('http://example.com/script.js')
 *   include('http://example.com/style.css')
 *   include('http://example.com/script.php', 'js')
 * @param {string} url URL of the CSS or JS file
 * @param {string} [type] 'css' or 'js' to specify the type of file.
 *                        This parameter is optional, but it should be 
 *                        used if the url does not end in '.css' or '.js'.
 */
const include = (url, type = url.split('.').pop().toLowerCase()) => new Promise((resolve, reject) => {
    let elem;

    if (type === 'css') {
        elem = document.createElement('link');
        elem.rel = 'stylesheet';
        elem.href = url;
    } else if (type === 'js') {
        elem = document.createElement('script');
        elem.src = url;
        elem.async = false;
    } else {
        throw new Error('Failed to include ' + url + ' due to unknown file type.');
    }

    elem.onload = resolve;
    elem.onerror = reject;
    document.head.appendChild(elem);
});