length of what now

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!