Validating a form with javascript function

Question

I am trying to validate a form using a JavaScript function. I think it works, but the output disappears almost immediately.

Here is the relevant code:

<form name="myForm">
    Your Age: <input type="text" name="age">
    <input type="submit" value=" Send " id="submit">
</form>



document.getElementById("submit").addEventListener("click", validate);

function validate () {

    var x = document.forms["myForm"]["age"].value;

    if (x > 0) {
        document.getElementById("demo").innerHTML = "Congrats you have entered x, your answer has been submitted";
    }
    else {
        document.getElementById("demo").innerHTML = "ERROR: Please enter a valid age";
        return false;
    }

}

What ya think boys?

"Boys?" If the output is disappearing I would guess the form submit is going ahead, causing the page to reload. Have you looked at event.preventDefault()? Also, it's generally better to validate in response to the form element's submit event rather than a button click. — nnnnnn
– nnnnnn, Commented Mar 24, 2017 at 5:49
"stops the default event of an element from happening" Isn't that what the return false; is for? Or that's what this guy on the tutorial i'm watching said. — Brixsta
– Brixsta, Commented Mar 24, 2017 at 5:50
yes, for instance if i input a string it replies, ERROR please enter a valid age and then dissapears — Brixsta
– Brixsta, Commented Mar 24, 2017 at 5:57
return false doesn't work from within an event handler bound with addEventListener(). (It does work from event handlers bound via older techniques, but those are generally frowned upon these days. Stick with .addEventListener() and use event.preventDefault().) — nnnnnn
– nnnnnn, Commented Mar 24, 2017 at 6:05

Sagar V · Accepted Answer · 2017-03-24 06:01:29Z

You have to prevent the default behaviour by getting the event argument in validate function and use .preventDefault(); to prevent default behaviour.

Then if the condition satisfies, submit the form via JavaScript using submit() function.
For that, either add the action and method or use ajax.

document.getElementById("submit").addEventListener("click", validate);

function validate (e) {
    e.preventDefault();
    var x = document.forms["myForm"]["age"].value;

    if (x > 0) {
        document.getElementById("demo").innerHTML = "Congrats you have entered x, your answer has been submitted";
        //submit your form here by using formelement.submit();
    }
    else {
        document.getElementById("demo").innerHTML = "ERROR: Please enter a valid age";
        return false;
    }

}

<form name="myForm">
    Your Age: <input type="text" name="age">
    <input type="submit" value=" Send " id="submit">
</form>
<div id="demo"></div>

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