You wouldn't believe me...

    ie-754
    //=> -754

Edit: I think this is a WTF WebKit

--- @rwaldron

Screenshot

Another Screenshot

eval() is a mysterious function. More so than people believe, hell its spec isn't even clear. And so i present... The two stages of eval denial.

Try out these functions with

    [foo("foo=2"),foo]

and take that path with me.

Tested on chrome, firefox ,opera, and safari.

Apparently eval is evil.

Doesn't seem so evil.

    function foo(x){var foo=1;eval(x);return foo;};

    //[2,function foo(x){var foo=1;eval(x);return foo;}]

Apparently assigning eval to a variable changes how it acts. WTF.

...Ok I can deal with that I guess.

    function foo(x){var foo=1, bar=eval;bar(x);return foo;};

    //and just returning it, not saving it

    function foo(x){var foo=1;(function(){return eval})(x);return foo;};

    //[1, 2]

You've probably seen this one here before. NaN (Not a Number) suffers from a terrible case of self-deception.

    typeof NaN
    //=> number

--- @3rdEden

Its still hilarious. Now, check this out:

    3..toString();
    // "3"

Explanation here.

(By way of the wtfjs twitter firehouse. Cheers: @devongovett, @getify, @mahemoff. @cramforce, @mathias!)

Look at me, I'm the magic increasing number!

    9999999999999999
    //=> 10000000000000000

--- @thomasfuchs

How do you determine if a number is an integer in JavaScript?

    x = 1;

    x === Math.floor(x);
    // returns true

But what happens if we try to add a method for this to the Number prototype?

    Number.prototype.isInteger = function() {
      return this === Math.floor(this);
    }

    x = 1;

    x.isInteger();
    // returns false!

Why? It turns out that when you add methods to Number, the type of the number inside the method becomes "object" rather than "number", but Math.floor returns a result of type "number". If you use the === operator, the two values are no longer equal because they're different types. So the method can be fixed two ways.

Solution 1 is to avoid comparing types:

    Number.prototype.isInteger = function() {
      return this == Math.floor(this);
      // works but breaks if you care about 0 vs other falsy values
    }

Solution 2 is better; cast "this" to the Number type and then the types are equal.

    Number.prototype.isInteger = function() {
      return Number(this) === Math.floor(this);
    }

--- @attaboy