The

document

object has methods that allow us to select the desired HTML element. These three methods are the most commonly used for selecting HTML elements

//finds element by id
document.getElementById(id) 

//finds elements by class name
document.getElementsByClassName(name) 

//finds elements by tag name
document.getElementsByTagName(name)

In the example below, the getElementById method is used to select the element with id="demo" and change its content

var elem = document.getElementById("demo");
elem.innerHTML = "Hello World!";

Note: The example above assumes that the HTML contains an element with

id="demo"

, for example

<div id="demo"></div>