Use the value property to get the value of a button in JavaScript.
Example
You can try to run the following code to find the value of a button −
<!DOCTYPE html>
<html>
<body>
<form id="myForm">
<button id="btn" type="button" value="my_button">My Button</button>
</form>
<script>
var str1 = document.getElementById("btn").innerHTML;
var str2 = document.getElementById("btn").value;
document.write("Button text: "+str1);
document.write("<br>Button value: "+str2);
</script>
</body>
</html>