The documentation for write() says:
Returns false to indicate that the kernel buffer is full, and the data will be sent out in the future.
and the documentation for the drain event says:
After a write() method returned false, this event is emitted to indicate that it is safe to write again.
What does that mean? Do I have to wait for the drain event before I can write again? What happened to the data which I tried to write? Is that lost? What happens when I call write without waiting for the drain event?
You can call write() without caring about the return value safely. Node will buffer any write calls while the kernel buffer is full, and push them in order as you would expect. You do not need to wait for the 'drain' event before writing again.
Optionally, you can check the return value of write() and then notify the thing that is writing to the Stream that the buffer is full. This is exactly what Stream#pipe() does.
So usually, just use Stream#pipe() and all the goodies are taken care of for you :)