Prevent onclick if input is empty

[files___]

Hi folks, 
I need an help. I have this button:
 

<form ... >
    <textarea ...></textarea>
    <button onclick="ajaxCart.add(product_id=3, attribute_id=13, true, null, quantity=1, null, stop);" type="submit" name="mymod_pc_submit_codice" class="button btn btn-default button-medium">
         <span>Aggiungi<i class="icon-chevron-right right"></i></span>
    </button>
</form>

 

It allows me to: 

1 - submit a form (with a textarea field)
2 - call the ajaxCart.add function when user click the button.

So I have one button for 2 "actions". My problem is: how can I stop the onclick function if the input is empty?! I can stop the submit just adding "required" on the textarea tag. But how can I do with the ajaxCart.add function?!

 

Thanks.

Edited April 27, 2017 by files___ (see edit history)