javascript on submit

document.getElementById("#myFormId").addEventListener("submit", function(e){
    if(!isValid){
        e.preventDefault();    //stop form from submitting
    }
  	//do whatever an submit the form
});

document.getElementById("myForm").submit();

document.forms["myform"].submit();

 <form onsubmit="myFunction()">
  Enter name: <input type="text">
  <input type="submit">
</form>