javascript formsubmit event code example

Example 1: js form submit listener

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

Example 2: prevent form submission on onsubmit function calls

returning false here won't be executed and the form will be submitted either way. You should also call preventDefault to prevent the default form action for Ajax form submissions.

function mySubmitFunction(e) {
  e.preventDefault();
  someBug();
  return false;
}

Get more references : https://stackoverflow.com/questions/3350247/how-to-prevent-form-from-being-submitted