To get the id of the form in JavaScript, use the id property.
Example
You can try to run the following code to learn how to get the id of the form in JavaScript.
<!DOCTYPE html>
<html>
<body>
<form id="myForm">
<button id="btn" type="button">My Button</button>
</form>
<script>
var str = document.getElementById("btn").form.id;
document.write("Form id of above button: "+str);
</script>
</body>
</html>