For me this is the best way:
<form id="myForm">
First name: <input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br><br>
<input type="button" onclick="myFunction()" value="Reset form">
</form>
<script>
function myFunction() {
document.getElementById("myForm").reset();
}
</script>