Validating Form Input

When to validate form input

• When text is typed, use an onChange event handler for each form element that should be validated
• When the form is submitted, use an onClick event handler for the button that submits the form

<script language="Javascript">
function qty_check(item) {
    if ((parseInt(item.value) >= 10) && (parseInt(item.value) <= 100))
        return true;
    else {
        alert("Please enter a number between 10 and 100");
        return false;
    }
}

function validateData(theForm) {
   if (qty_check(theForm.quantity)) {
      alert("Order has been Submitted");
      return true;
   }
   else {
      alert("Sorry, Order Cannot Be Submitted!");
      return false;
   }
}
</script>
...
<form name="order_items" action="somewhere.html" method="post">
How many items do you need? 
  <input type="text" name="quantity" onChange="return qty_check(this);">
  <br> 
  <input type="submit" value="Enter Order" onClick="return validateData(this.form);">
</form>