[felgall]

You have:

xhr.onreadystatechange = setWord();

which runs setWord are attaches whatever is returned to be run whenever the readyState changes. At this point it is 0 which is not equal to 4.

You don't return anything from the setWord function and so there is nothing to actually run when the state does change to 1, 2, 3 and eventually 4.

To get setWord to run when the state changes you need to change that line to read:

xhr.onreadystatechange = setWord;

That way the function will not run now and will instead run when the state changes.