This code sets an event listener on the window object for the keydown event. That means when the window has focus (or any child thereof) and a keyboard key is pressed that handler will intercept the event.

On keyboard events there is a key property which names the keyboard key pressed to result in the event execution. This handler is only looking for the 4 arrow keys and modifying an object assigned to variable inputDir. What’s interesting is that inputDir is reassigned to a baseline on each key press irrespective of the keyboard key.

Objects in JavaScript are what other languages call a hash map, a key/value pair. Key names are always string data types and there is no type or value restriction on object values. This particular object has two keys assigned: x and y. Values are reassigned respectively to an arrow key press.