JavaScript Automatic Type Conversion Confusion

Question

Given this script:

var number = NaN;
if (!number) {
 alert("yes");
}
alert(number == false);

Why does the first expression `!number evaluates to true, while the second expression number == false evaluates to false?

http://jsfiddle.net/8EWG4/

• Ivan: I'm guessing this has to do with NaN being 'falsey'.

Answer 1

Keep a look in this article: http://www.smashingmagazine.com/2011/05/30/10-oddities-and-secrets-about-javascript/

There's some tricks about javascript, including informations about NaN:

NaN is a Number

You thought null being an object was ridiculous? Try dealing with the idea of NaN — "not a number" — being a number! Moreover, NaN is not considered equal to itself! Does your head hurt yet?

alert(typeof NaN); //alerts 'Number'
alert(NaN === NaN); //evaluates false

In fact NaN is not equal to anything. The only way to confirm that something is NaN is via the function isNaN().

• pimvdb: You can actually use that property as a way of checking for NaN - if x !== x yields true, then x must be NaN.

Answer 2

The ECMAScript specification says so:

x == y is defined as (11.9.3):

If x is NaN, return false.

And ! calls ToBoolean (9.2) first (and then returns the opposite):

The result is false if the argument is +0, −0, or NaN; otherwise the result is true

Answer 3

To evaluate a variable is NaN ( not a number ), consider using isNaN(number) . It will give you correct answer.

Answer 4

From wikipedia - JavaScript syntax, boolean:

When used in a logical context, 0, -0, null, NaN, undefined, and the empty string ("") evaluate as false due to automatic type coercion.

!NaN == true

So, as NaN is coerced to false, !NaN evaluates to true.

However, NaN is not equal to false - it is a different type/value.

• helpermethod: Then why does alert("" == false); evaluate to true?
• pimvdb: @Oliver Weiler: That's also in the specification. == should call ToNumber on both arguments in case of a string and a boolean, and ToNumber returns 0 for both "" and false.