Ideas on a testing focussed code transform

# Pranay Prakash (2 months ago)

Hey all,

I was just reading the docs for redux-saga where I encountered a nice design pattern for a saga which is (correct me if I'm wrong) a regular javascript generator function that yields the intent to call a function (instead of actually calling a function) until it goes through all the steps. If that doesn't make sense, consider this simple function:

async function findFriends() {
  const myId = getMyID();
  const myUser = await fetchUser(myId);
  return myUser.friends;

instead of actually making the function calls needed, we can instead have a function that does something like this:

function* findFriends() {
  const myId = yield { fn: getMyID };
  const myUser = yield { fn: fetchUser, args: [myId] };
  return myUser.friends;

This is a pure generator function that doesn't actually do anything, but has all the necessary information to recreate the original function[1] (or have a library "trampoline" through the function and make all the necessary calls for you)

A HUGE plus of the second version of the function is that it's easily testable (unlike the first one). Pure functions are easier to test. Testing this is simply a matter of calling .next(), getting the "intent", making sure the right intent was yielded (good enough for unit testing) and calling .next() again with the "mock" value, you want to return and continue to test.

An observation to make here is that you can transform the original version of this code (easy, normal code to write) to the latter (easy code to test). So, what about having some sort of babel transform perhaps that can convert the first to the second but only in the context of unit tests. You write code the normal way as you would for your application and don't worry about test suite implementation details (mocking/dependency injection/etc.), and when you want to test, simply import your function (which gets converted to a generator) and step through it to test different scenarios.

I personally think this is a super clean way to do testing since the tests never interfere with how you actually write the code AND you don't have to explicitly mock.

Does anyone have thoughts on this / prior research (or knows about an existing implementation of this)?

# Isiah Meadows (2 months ago)

You're describing a variant of the interpreter pattern. I've used similar in task scheduling contexts, and the key drawback is you no longer have useful stack traces.

I haven't done any formal research on the approach, though. I will caution you that async functions are already widely used enough (particularly in Node) they're unlikely to go anywhere beyond something drastic.

# Pranay Prakash (2 months ago)

Hmm, I suppose the stack traces drawback doesn't matter as long you're only using this within the context of testing. The emphasis here is that the actual application code that you run in production is what you wrote (the imperative function calls), but when you 'require' the function into your testing code, it gets transformed into this pure generator function that's easier to test.

I don't see how the state/future of async functions affects this. In fact, in my example, I use an async function to try and show that this transform can even work for async functions (and will convert them to a synchronous generator function rather easily)

# Isiah Meadows (2 months ago)

I think what you're looking for is dependency injection, which is simply the functions you care about passed as an argument rather than sent from yield. In your case, the injections would be recording what method is called when and with what, but that's what I typically do. As an added bonus, I don't need to rely on how the functions are structured - I can even record actions done from iterables. (That's one area yours won't work.)

The only catch is that you can't asynchronously block, but that's far more rare in practice (automated testing doesn't need it, and you can set breakpoints when debugging, removing the need to asynchronously hook into it).

# Pranay Prakash (2 months ago)

I don't understand what you mean by "record actions done by iterables". Could you please expand on that :)

My main reason for being weary of dependency injection is that I don't want to change how I write application code just to make it more testable. That's a pretty well shared concern about dependency injection whereby it affects the entire structure of your codebase, or adds additional arguments to simple functions for the sole purpose of testing it.

With the method I'm proposing, you wouldn't have to change how you code at all, and I think that's really nice

# Isiah Meadows (2 months ago)

You're still changing code as evidenced by your original email.

  • With dependency injection, you're changing the call sites (name -> method

call) and argument count or state size (to hold the injection).

  • With your idea, you're changing the call sites (name+args to yield) and return value (value -> yield result).

To give you an idea what I mean by dependency injection, take a look at this:

I mean small functional dependency injection, not the boilerplatey Java-style one. Mine is a bit more complex, because I also am passing shared state with it (like parsed args and config), but it's just an extra object property on that state, as defined in the State constructor in lib/cli/run.js.

# Pranay Prakash (2 months ago)

Oh, I'll check out those links. However, the key part of my idea is to NOT change the way write application code.

So, when I described the second version of the codebase (the one with yields), that's not how I want you, the developer, to write code. I simply propose a static code transform that convert the code you originally wrote (first example) to the pure version with yield calls.

The trick here is, the original version of the code will be used when you test your program, but the transformed code is what gets used solely when you're testing (meaning your tests have to account for the code being transformed, but I can image a good library to abstract some of those details away)

I'll have to read up more about the dependency injection links you sent before I can comment further on that comparison between this idea and dependency injection

# dante federici (2 months ago)

To add to this --- a Monad is a useful concept for separating the description of execution versus actually running the execution:

# Pranay Prakash (2 months ago)

Yup, that applies perfectly to what I'm describing Dante (and is some great reading) :)