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It teaches you the funky behavior of JavaScript. For instance, `[]["filter"]` returns an empty function called filter.

In order to get particular characters (for example: f), the script uses "false"[0], where "false" is derived from adding ![] + [], and 0 is derived from +[].

Putting all of that together ($ node):

> (![]+[])[+[]]

'f'

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Not sure that []["filter"] is all that funky of an example as it's just grabbing the arrays prototype method "filter" using bracket notation.

Here's some better examples of JS oddities: http://stackoverflow.com/q/9032856/1538708

The funkiness is really that []["filter"]["constructor"] is a function that behaves like eval(), or rather, it returns a function that when called executes the string that was passed to the constructor function. That's how they're actually running the code.

> []["filter"]["constructor"]("alert(1)")()

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