use appendChild to add html content 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: javascript append to paragraph

// In the JS script

var parElement = document.getElementById("myPar");
var textToAdd = document.createTextNode("Text to be added");
parElement.appendChild(textToAdd);

//In the HTML file

<p id="myPar"></p>