Question

I got an issue that negation of zero produces false in my JavaScript code.
I have simplified the code to demo the issue as given below.

<input id="iid" value="0" />

<script type="text/javascript">
  zero = document.getElementById('iid').value;  
  alert( ( !zero ? 'true' : 'false' ) );  // alert message is "false".
</script>

Why negation of zero become false?

Answer 1

You’re negating the string "0". Any string becomes false when negated except the empty string:

!0       true
!"0"     false
!""      true
!+"0"    true

The last expression is true because the + operator converts the string into a number.

An input value is always a string, which makes sense semantically as well because it’s a combination of characters entered by the user. If you want to interpret is as a number you’ll have to convert it into one yourself.