Question

This one says false, meaning the "" is a number:

alert(isNaN("")); 

This one says NaN, meaning the "" is not a number and cannot be converted:

alert(parseFloat(""));

I was expecting the second code to convert "" to 0 since "" is a number when tested in IsNaN but I was wrong! Am I getting crazy or I just missed something?

Answer 1

parseFloat tries to parse a number from string where as isNaN converts the argument to number before checking it:

Number("") //0 http://ecma-international.org/ecma-262/5.1/#sec-9.3.1
parseFloat("") //NaN http://ecma-international.org/ecma-262/5.1/#sec-15.1.2.3

Apparently this is “broken” or “confusing”, so from the specs:

A reliable way for ECMAScript code to test if a value X is a NaN is an
expression of the form X !== X. The result will be true if and only if
X is a NaN.

0 !== 0 // false
NaN !== NaN //true

function isExactlyNaN(x) {
    return x !== x;
}