Why does `new Array(4).join(""+10) + " Batman!"` return "NaNNaNNaNNaN Batman!" : <p>I saw this snippet in a video online (if anyone can find the link, please feel free to add it to the question). </p>

<p>The video was quite short, so there was no explanation as to why JavaScript returns this really random string.</p>

<p>This probably has something to do with how JavaScript handles certain types...</p>
