To set the indentation, use the textIndent property.
Example
You can try to run the following code to return the indentation of the first line of text with JavaScript −
<!DOCTYPE html>
<html>
<body>
<div id = "myText">
This is demo text. This is demo text. This is demo text. This is demo text. This is demo text. This is demo text.
</div>
<br>
<button onclick = "display()">Set Text Indent</button>
<script>
function display() {
document.getElementById("myText").style.textIndent = "50px";
}
</script>
</body>
</html>