Let's learn another shortcut.
You already know about statements like this:
score = score + 1;
alien_count = alien_count - num_killed;
width = width * 2;
And we've
also met this short-cut:
score++; // same as score = score + 1
You
can also express it like this:
score+= 1; // same as score = score + 1
But more
interestingly (and more usefully):
alien_count -= num_killed; // same as alien_count = alien_count - num_killed
width *= 2; // same as width = width * 2;
These shortcuts can save typing, and also may reduce bugs because there's less chance of mistyping a variable name.
However, we believe that while they're easier to write, for learner-coders they're harder to read, and so you won't see this syntax too often in S2JS examples. But feel free to use it yourself.