Stepan ParunashviliTwitter

Crafting Functions

We write so many functions in our programs that they become second nature before we know it. Like ants in a colony, they are numerous beyond imagination and they come together to form some surprisingly complex systems.

It begs the question: how do we write good functions? It can seem trivial: they’re just like ants after-all. But there is leverage in the answer: the right decisions multiply throughout your codebase and bubble up into great design.

I think there are about three key ideas you can employ to craft good functions. I wanted to share them with you.


Let’s start with an example. We have an app, and we want to export some data in a JSON format. Here’s what a function for that could look like:

function exportFile() { 
  try {
    const data = getData(); // [Data, Data, Data]
    const exportableData = toExportableData(data); // ExportableData
    const jsonStr = JSON.stringify(exportableData); // '{"data": {...
    const fileURL = saveFile("export.json", jsonStr); //
  } finally {

Seems straight forward: To export as JSON, we first get our data. Now, this data may have some sensitive info, so we clean that up and transform into something exportable; ExportableData. Once we have that, we get a string representation, save the file, and badabing, badaboom, we’re done.

Okay, we’ve got something working well.

A new requirement

But life moves on and our program needs to evolve. Instead of just exporting JSON, we need to do more: we also need to export a CSV file.

How do we do that?

The first thing we notice, is that exporting a CSV is very similar to exporting JSON. Can we abstract exportFile?

Idea 1: Configuration

One thing we can do, is to introduce a new flag: something like exportFile(/*isCSV=*/ true)

function exportFile(isCSV) { 
  let fileURL
  if (isCSV) { 
    const csvStr = toCSVStr(exportableData)
    fileURL = saveFile("export.csv", jsonStr);
  } else { 
    const jsonStr = JSON.stringify(exportableData);
    fileURL = saveFile("export.json", jsonStr);

By introducing this flag, we can conditionally produce a different fileURL: one for CSV and one for JSON. With that we see the first concept for abstraction: configuration. You pass some configuration, and you leave it to your function to figure what to do.

So, is it a good idea?

The key advantage is that our logic is centralized.

With configuration, the caller is limited in what they can do: they can only provide flags. All the true logic stays inside exportFile. This means that callers of the function can’t go crazy and do something unsupported. And that could give us some peace of mind.

The key disadvantage is that…our logic is centralized.

This will work, but let’s think about it. First, notice that in order to understand exportFile now, we need to understand both the CSV and JSON case. Imagine if someone opens up exportFile to figure out what it does: if they only cared about JSON, they now have to understand more logic than they needed. Anyone who changes the logic for CSV, may also end up breaking JSON. **exportFile** has become complected.

Notice also, that because the caller of this function can only provide flags, their hands are tied for use-cases that you didn’t support. This was supposed to give you peace of mind, but it certainly can frustrate callers. imagine if they wanted to support XML, what could they do? They’d have to edit exportFile to support this case. (God forbid they edit it to be something like exportFile(isCSV, isXML) — now you have invariant conditions on your hands). By being so specific, you’ve chosen to make your function less abstract — this of course means that it is less powerful. **exportFile** has become hard to extend

For better or worse, configuration gives the caller the least amount of power

If you imagine a sort power spectrum, where the caller has the least power on the left, and most power on the right, configuration would be on the left. You control what the caller does so tightly that it gives your certainty, but makes your function more complex and less useful.

Say you wanted to address the problems, and move to the right of this spectrum, what could you do?

Idea 2: Inversion

Well, if you look at what we wrote, we can notice that the only part that is really different, is the bit about taking exportData, and creating a fileURL.

const exportableData = toExportableData(data); // ExportableData
... // *This can be different! Somehow we need to get a fileURL* 

So one thing we can do is this: instead of providing a flag, we can provide a function:

function exportFile(exportableDataToFileURL) { 
  try {
    const data = getData(); // [Data, Data, Data]
    const exportableData = toExportableData(data); // ExportableData
    const fileURL = exportableDataToFileURL(exportableData)
  } finally {

Now, for JSON, we can write

exportFile((exportableData) => { 
  return saveFile("export.json", JSON.stringify(exportableData));

and for CSV we can write:

exportFile((exportableData) => { 
  return saveFile("export.csv", toCSVStr(exportableData));

Oky doke, this is cool.

The key advantage is that you give the caller more power

With this we solve both of the problems we had with configuration. Now if someone looks under the hood at exportFile, they won’t see unrelated code about csv. If they wanted to extend to XML, they can simply provide a different function. We’ve given the caller much more power

The key disadvantage is that it can be either too powerful or not powerful enough

We’ve abstracted further, but there is a price there. The first is, that we think we know that what we really need to pass outwards is exportableData, and what we need to return is a fileURL. What if we were wrong? For example, some may need a slightly different data format — instead of exportableData they need someOtherKindOfExportableData. By the time we figured that out, it’s possible that there are numerous new usages of exportFile, which we’ll have to support as we evolve this function.

One way we could have prevented this, is to have stuck with configuration. This way, anyone who wanted to support something would have to funnel through this function, which would give us time to think about what the best abstraction was.

Another way, would have been if this function was abstracted even further, so callers could have easily supported someOtherKindOfExportableData.

Inversion lies in the middle of the power spectrum

Inversion is more powerful than configuration, but it’s not the most powerful method. This can be a great choice, but you risk either being too powerful and exposing errors, or not being powerful enough and restricting callers.

We know the less powerful option: configuration. What would the most powerful one look like?

Idea 3: Composition

The next thing we may notice, is that our exportFile function is actually built up some building blocks that could be useful for a bunch of different things. For example, many functions may want a loading state, or just need to get exportableData, etc. We could create those building blocks:

function exportJSONFile() { 
  withLoading(() => saveJSONFile(getExportableData()))

function exportCSVFile() { 
  withLoading(() => saveCSVFile(getExportableData()))

The key advantage is that the user gets the most power

The building blocks that we just built, can be used in a myriad of ways. The user can support CSV, XML, can use isLoading with some other function, and choose to provide a different kind of exportableData. We’ve provided a lot of power for the user.

The key disadvantage is that you are the most vulnerable to mistakes

The disadvantage though, like in the case of inversion, is that we open ourselves up to a lot of mistakes. What if isLoading was really meant for files, and other things should have been using a different flag? What if people start using saveJSONFile, and pass data that wasn’t really an export? These are all cases that we have implicitly allowed with our abstractions.

There’s a further problem: notice that with our first example of exportFile, you the code was more concrete: you could see what was actually happening. When code is more abstract, it’s a bit harder to reason about what is actually happening. Now, it can be worth it for the power gains, but if you optimized prematurely, you’re just paying this price for nothing. An example of this unnecessary price is saveJSONFile and saveCSVFile — if we had inlined those, the overall composition would still be abstract but more understandable. These are the kind of things to watch out for as you abstract at this level.

Composition is at the end of the spectrum

And with that, we see that composition gives us the most power, but gives us the most opportunities to shoot ourselves in the foot. Boy can it be worth it though.


It’s funny to notice that with each option, the pro is the con. So how do we pick? I think one heuristic you can use is this: pick the most powerful option you can limited by your confidence. For example, if you have a light understanding of the problem, stay on the lower side of the abstraction spectrum. As you understand more (say, time to introduce XML) you can evolve to the powerful side of the spectrum. When you’re very confident, and you can see good use-cases for your building blocks, lean to the most powerful side of the spectrum.

Thanks to Daniel Woelfel, Alex Reichert, Julien Odent for reviewing drafts of this essay

Thoughts? Reach out to me via twitter or email : )