javascript detect form submit 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: how to run js before submit html

<form onsubmit="return do_something()">

function do_something(){
   // Do your stuff here
   return true; // submit the form

   return false; // don't submit the form
}