isNaN('Infinity'); // false

document.write(isNaN("")) // false
document.write(isNaN(" "))  // false
document.write(isNaN(0))  // false
document.write(isNaN(null)) // false
document.write(isNaN(false))  // false
document.write("" == false)  // true
document.write("" == 0)  // true
document.write(" " == 0)  // true
document.write(" " == false)  // true
document.write(" " == 0)  // true
document.write(0 == false) // true
document.write(" " == "") // false

" " == 0 == false

"" == 0 == false

"" != " "

alert(isNaN(parseInt("   ")));

alert(isNaN(parseFloat("    ")));

alert(' ' * 1); // 0
alert('x' * 1); // NaN

var isNumber = function isNumber(value) {
   return typeof value === 'number' &&
   isFinite(value);
}

function isInteger(s)
{
   return Math.ceil(s) == Math.floor(s);
}