In ActionScript (NaN==parseFloat(input.text)) warns that it will always be false. Why?

Question

Despite the rather clear documentation which says that parseFloat() can return NaN as a value, when I write a block like:

if ( NaN == parseFloat(input.text) ) {
  errorMessage.text = "Please enter a number."
}

I am warned that the comparison will always be false. And testing shows the warning to be correct.

Where is the corrected documentation, and how can I write this to work with AS3?

Answer 1

Because comparing anything to NaN is always false. Use isNaN() instead.

Answer 2

isNaN(parseFloat(input.text))

Answer 3

BTW, if for some reason you don't have access to isNaN(), the traditional method is to compare the number to itself:

if( number != number )
{
    //Is NaN 
}