Method to add one or more function handler on an element's onload event...

addOnloadHandler.js

/**
 * Method to add one or more function handler on an element's onload event
 * This is because onload cannot carry out many separate functions.
 * 
 * @param {any} element DOM element
 * @param {any} handler function
 */
const addOnloadHandler = (element, handler) => {
    if (typeof element.onload === 'function') {
        const pre_func = element.onload;
        element.onload = () => {
            pre_func();
            handler();
        }
    }
    else {
        element.onload = handler;
    }
}