add css by javascript code example

Example 1: how to change style of an element using javascript

<html>
<body>

<p id="p2">Hello World!</p>

<script>
document.getElementById("p2").style.color = "blue";
</script>

<p>The paragraph above was changed by a script.</p>

</body>
</html>

Example 2: how to give css style in javascript

document.getElementById("myH1").style.color = "red";

Example 3: change color of css in js

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Change the Background Color with JavaScript</title>
<script>
    // Function to change webpage background color
    function changeBodyBg(color){
        document.body.style.background = color;
    }
    
    // Function to change heading background color
    function changeHeadingBg(color){
        document.getElementById("heading").style.background = color;
    }
</script>
</head>
<body>
    <h1 id="heading">This is a heading</h1>
    <p>This is a paragraph of text.</p>
    <hr>
    <div>
        <label>Change Webpage Background To:</label>
        <button type="button" onclick="changeBodyBg('yellow');">Yellow</button>
        <button type="button" onclick="changeBodyBg('lime');">Lime</button>
        <button type="button" onclick="changeBodyBg('orange');">Orange</button>
    </div>
    <br>
    <div>
        <label>Change Heading Background To:</label>
        <button type="button" onclick="changeHeadingBg('red');">Red</button>
        <button type="button" onclick="changeHeadingBg('green');">Green</button>
        <button type="button" onclick="changeHeadingBg('blue');">Blue</button>
    </div>
</body>
</html>

Example 4: js add css style to element

// Create our stylesheet
var style = document.createElement('style');
style.innerHTML =
	'.some-element {' +
		'color: purple;' +
		'background-color: #e5e5e5;' +
		'height: 150px;' +
	'}';

// Get the first script tag
var ref = document.querySelector('script');

// Insert our new styles before the first script tag
ref.parentNode.insertBefore(style, ref);