Create a DOM element if it does not exist already

create_or_get_element_by_id

/**
 * Create a DOM element if it does not exist already
 *
 * @param {String} id the id of the element
 * @param {String} eltTag HTML tag of the element
 * @param {String} txtContent = '' the content of the element
 * @return {DOM element} the DOM element
 */
function createOrGetElementById(id, eltTag, txtContent = '') {

	var elt = document.getElementById(id);
	
	if (elt === null) {
		elt = document.createElement(eltTag);
		elt.id = id;
	}
	if (txtContent)
		elt.appendChild(document.createTextNode(txtContent));

	return elt;
}