what does 'in' mean in javascript if statement : <p>Hi i'm following a tutorial learning to use javascript. </p>

<pre><code>function move(keyclick) {
if(40 in keyclick) {}
playerMove.x++;
render(); }
</code></pre>

<p>What does the 'in' word mean? I understand what the function is doing, but why not just use == 
?</p>

<p>Thanks </p>
