Deep Streams

Usually when I think of Node, I think about web servers.  That’s mostly what I use it for when writing code- setting up a simple test server for a proof of concept, or bringing in Express and its ecosystem for more production-ready projects.

But in reality, I use Node for a whole lot more.  Of course, just about anything NPM related is a use of Node- but it also powers all the awesome developer tools that we don’t even really need to think about much anymore.  When a minifier runs over your Javascript code before you push to production- Node is probably doing that magic.  Same for a bundler.  It’s behind the scenes on quite a bit of frontend workflow these days.  I use it for such all the time, but I hadn’t had much chance to really write any of those dev tools until recently.

The task was pretty simple- I had a js file with a bunch of arrays and objects someone else had entered.  The formatting had been mangled somewhere along the way- there were long strings of spaces everywhere- I wanted to strip them out, but leave any single spaces (to preserve multi word strings).  Now I know: any halfway good code editor will have the search/replace feature to handle this, but I could see this being a nice little utility to write in Node.  That way, I could run it over an entire directory if necessary (probably won’t ever be necessary, but I really wanted to do this short little side project).

My first iteration was a super simple script using the fs module.  First, a nice path splitter utility in a separate file.  This takes a string, splits it out by path and extension, and inserts ‘-‘ and whatever new string you want.  This prevents overwriting the original file (though this part would be unnecessary if you do want to overwrite- just pass the same file name to the write function):

Then we can use that in our script to strip multi-spaces and return a new file:

All very cool. But I really like the notion of streams in Node. What if the file I need to manipulate is really large? With my current setup, it might take a lot of memory to read the entire file, then write it out. But that’s what streams are for! So I rewrote the script with a custom transform stream. It wasn’t all that difficult- as soon as I realized that the required method on your custom stream (that extends the Transform class) has to be named _transform. If you leave out the underscore, it will not work (recognizes Transform as undefined).

Again, in a separate file (small modules for the win!), I defined my custom stream:

Then it was just a matter of importing that and the path splitting utility created for the original fs version (code reuse for the win!) and running a couple Node-included streams (createReadStream and createWriteStream) that can make a stream from a file automatically:

Both methods (fs and stream) are simple and concise. Both have their place. Creating a custom transform stream was probably unnecessary for this task, but would be very useful for very large files. Either way, it was a fun quick dive into some other corners of Node!