JavaScript can change HTML content

One of many JavaScript HTML methods is getElementById().

This example uses the method to "find" an HTML element (with id="demo") and changes the element content (innerHTML) to "Hello JavaScript":

Example: document.getElementById("demo").innerHTML = "Hello JavaScript";

JavaScript accepts both double and single quotes:

Example: document.getElementById('demo').innerHTML = 'Hello JavaScript';

JavaScript Can Change HTML Styles (CSS)

Changing the style of an HTML element is a variant of changing an HTML attribute:

Example document.getElementById("demo").style.fontSize = "35px";