- The isNaN() function returns a boolean -true or false- after checking if a value is numeric or not. "NaN" stands for "Not a Number". NaN is also a value, returned when the parseInt() or parseFloat() functions (for example) can't extract a number from another data type (e.g., string).
- Best practice is to use isNaN() in conjunction with methods or custom scripts that should return a number. This snippet checks if the value entered by a user in a text field is numeric:
var age = parseInt( document.getElementById("age").value ); // This one ↑
if( isNaN( age ) ) { // true, so not a valid number... Click to run ->
alert( "Please enter a valid age!" );
document.getElementById("age").focus();
} else
alert( "Thanks for confirming your age" );Tip: a blank field (no value supplied) will also return NaN. If a user enters fractional age (like 21.5 for 21 and a half years old), JavaScript will have "21" as age, because we used parseInt().
- The negative is a bit confusing at first: just remember that if you get "true" back, it's not a number. An "isNumber()" function would be more intuitive, and you can write your own in a single line!
function isNumber(num) { return !isNaN(num) } // true if you passed a numberNote: the exclamation point ! ("unary negation operator") returns the opposite boolean value.
- The PHP equivalent to JavaScript's isNaN() is the is_numeric() function. The code below returns false, but would return true if assigned the text "10" (string) as value ( $age='10'; ).
$age = '10 years old';
if( is_numeric( $age ) )
echo 'Thanks for entering your age!';
else
echo 'Please enter a valid age.';Tip: PHP also includes the is_int() and is_float() functions, if you need more detailed info!
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