Let's take the last post a step further:

    console.log( (![]+[])[+[]]+(![]+[])[+!+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]] ) // "fail"

Breaking that mass of symbols into pieces we notices, that the following patten occurs often:

    console.log( (![]+[]) ) // "false"
    console.log( ![] ) // false

So we try adding [] to false. But through a number of internal function calls( binary + Operator -> ToPrimitive -> [[DefaultValue]]) we end up with converting the right operand to a String:

    console.log( [].toString() ) // ""
    console.log( (![]+[].toString()) ) // false + ""

Aha, so we are concatenating strings here! Now this is plain obvious! Moving on to the next bit:

    console.log( [+[]] ) // [ 0]
    console.log( +[] ) // 0

Thinking of a String as an Array we can access its first character via [0]. So "false"[0] returns "f".

I think you got the idea now and can figure out the rest by yourself!

Thanks to Mathias Bynens for pushing this out int the web.

This made me laugh. Out loud even!

    console.log((!+[]+[]+![]).length);
    // 9

Huh, wtf?! Lets see what we're getting the length of...

    console.log((!+[]+[]+![]));
    // "truefalse"

Ah! lulz. thanks to @stepiiik and @DavidGrudl!

Old friend (and clearly damaged by a career of JavaScript programming) @jakedevine threw this down on twitter a few days ago.

    var Z = "constructor"; 
    Z[Z][Z]("alert('wtfjs!')")();
    // alerts wtfjs!

Ok. So wtf exactly is happening here?

    Z[Z]
    // function String() { [native code] }

    Z[Z][Z]
    // function Function() { [native code] }

AHA! The constructor property of a string is String and the constructor property of String is Function. Makes sense.

    Z[Z][Z]("console.log('new Function accepts string for eval as argument')")();
    // new Function accepts string for eval as argument

Of course. One wonders, as usual, wtf the programmer was up to when he discovered this!

--- @brianleroux

Void is a blackhole...

    var void = function () {}
    //=> SyntaxError: missing variable name

-- @rwaldron

When you create instances of String or Number, they take the default value ("" for strings and 0 for numbers). This is not the same for Object and Array.

    var a = new Number;
    a == 0 // true

    var a = new String;
    a == "" // true

    var a = new Object;
    a == {} // false

    var a = new Array;
    a == [] // false

This is even more confusing when using the JSON-style syntax to create objects and arrays.

    var a = {};
    a == {} // false

    var a = [];
    a == [] // false

-- @remi

(Never forget: ==== - @brianleroux)