append in body in js code example

Example 1: add element to body javascript

var elem = document.createElement('div');
elem.style.cssText = 'position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000';
document.body.appendChild(elem);

Example 2: js add more text to element

document.getElementById("p").textContent += " This is the text from javascript.";

<p id ="p">This is the text from HTML.</p>