Javascript code is not catching null value

Question

I'm really new to javascript and I'm trying to prevent a textbox from accepting letters and empty values. This is what I have:

function checkInp()
{
var x=document.forms["newPerson"]["ThisBox"].value;
    if (isNaN(x) || is_null(x)) 
    {
        alert("This box can only contain numeric values");
        return false;
    }
}

Here is my form tag:

<form name="newPerson" action="newPerson2.php" method="POST" onsubmit="return checkInp();">

This DOES prevent letters from passing through, but it DOES NOT prevent empty values from going through.

Remember undefined is not === null

ffflabs
– ffflabs

2015-11-19 02:33:32 +00:00
Commented Nov 19, 2015 at 2:33 — ffflabs
– ffflabs, Commented Nov 19, 2015 at 2:33

Anik Islam Abhi · Accepted Answer · 2015-11-19 02:34:39Z

3

Try to check like this

if(!x || isNaN(x)){
  alert("This box can only contain numeric values");
  return false;
}

In javascript "",null,undefined,NaN,0 all consider false.

answered Nov 19, 2015 at 2:34

Anik Islam Abhi

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1 Comment

jaggedsoft Over a year ago

I recommend this approach as well. This is called falsey meaning "Something which evaluates to FALSE." Read more: james.padolsey.com/javascript/truthy-falsey

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