JavaScript form error state does not stay on same page

Question

I'm doing a simple validation for a form. The JavaScript validation works fine and the pop up alert box appears with the correct errors, BUT, upon clicking "Ok", I get re-directed to the next page. Ideally, it is supposed to stay at the same page so that the user can amend his/her mistakes.

This is a school project.

<script type = "text/javascript">

function show_alert() {
    if (document.getElementById('time1').value == document.getElementById('time2').value) alert("ERROR! You cannot book the same timing twice!")
    else if (document.getElementById('time1').value == document.getElementById('time3').value) alert("ERROR! You cannot book the same timing twice!")
    else if (document.getElementById('time1').value == document.getElementById('time4').value) alert("ERROR! You cannot book the same timing twice!")
    else if (document.getElementById('time1').value == "0") alert("ERROR! You cannot leave the first time slot blank!")
    else {}
} 
</script>

Answer 1

This should be easy to fix. In the onsubmit method of your form tag, do this:

<form onsumbit="return show_alert();">

Instead of

 <form onsumbit="show_alert();">

Without the return part, the script will run, and the form will be submitted anyhow. Also, if there is an error condition in the script, you need to add a return false; otherwise the form will still be submitted, and return true; if there is no error. You can edit the script like so:

<script type = "text/javascript">

function show_alert() {
    if (document.getElementById('time1').value == document.getElementById('time2').value) {
        alert("ERROR! You cannot book the same timing twice!");
        return false;
    } else if (document.getElementById('time1').value == document.getElementById('time3').value) {
        alert("ERROR! You cannot book the same timing twice!");
        return false;
    } else if (document.getElementById('time1').value == document.getElementById('time4').value) {
        alert("ERROR! You cannot book the same timing twice!");
        return false;
    } else if (document.getElementById('time1').value == "0") {
        alert("ERROR! You cannot leave the first time slot blank!");
        return false;
    } else {
        return true;
    }
} 

</script>