Asynchronous control flow with streams

Going through the examples that we presented so far, it should be clear that streams can be useful not only to handle I/O, but also as an elegant programming pattern that can be used to process any kind of data. But the advantages do not end at the simple appearance; streams can also be leveraged to turn asynchronous control flow into flow control, as we will see in this section.

Sequential execution

By default, streams will handle data in a sequence, for example, a _transform() function of a Transform stream will never be invoked again with the next chunk of data, until the previous invocation completes by executing callback(). This is an important property of streams, crucial for processing each chunk in the right order, but it can also be exploited to turn streams into an elegant alternative to the traditional control flow patterns.

Some code is always better than too much explanation, so let's work on an example to demonstrate how we can use streams...