Sunday 5 September 2021

Testing: reasoning over "testing implementation detail" vs "testing features"


I can't see how this article will be a very long one, as it only really dwells on one concept I had to reason through before I decided I wasn't talking horseshit.

I'm helping some community members with improving their undertanding of testing, and I was reviewing some code that was in need of some test coverage. The code below is not the actual code - for obvious "plausible deniability" reasons - but it's pretty much the same situation and logic flow we were looking at, just with different business entities and the basis for the rule has been "uncomplicated" for the sake of this article:

function validateEmail() {
    if ((isInstanceOf(this.orderItems, "ServicesCollection") || isInstanceOf(this.orderItems, "ProductsCollection")) && !isValid("email", this?.email) ) {
        var hasItemWithCost = this.orderItems.some((item) => item.cost > 0)
        if (hasItemWithCost) { // (*)
            this.addError("Valid email is required")

// (*) see AdamT's comment below. I was erroneously calling .len() on hasItemWithCost due to me not having an operational brain. Fixed.

This function is called as part of an object's validation before it's processed. It's also slightly contrived, and the only real bit is that it's a validation function. For this example the business rule is that if any items in the order have a cost associated with them; then the order also needs an email address. Say the receipt needs to be emailed or something. The real world code was different, but this is a fairly common-knowledge sort of parallel to the situation.

All the rest of the code contributing to the data being validated already existed, we're just got this new validation rule around the email not always being optional now.

We're not doing TDD here (yet!), so it's a matter of backfilling tests. The exercise was to identify what test need to be written.

We picked two obvious ones:

  • make an orderItems collection with no costs in it, and the email shouldn't be checked;
  • make an orderItems collection with at least one oneitem having a costs in it, and the email should be present and valid;

But then I figured that we've skipped a big chunk of the conditional logic: we're ignoring the check of whether the collection is one of a ServicesCollection or a ProductsCollection. NB: unfortunately the order can comprise collections of two disparate types, and there's no common interface we can check against instead. There's possibly some tech debt there.

I'm thinking we have three more cases here:

  • the collection is a ServicesCollection;
  • the collection is a ProductsCollection;
  • the collection is neither of those, somehow. This seems unlikely, but it seems it could be true.

Then I stopped and thought about whether by dissecting that if statement so closely, I am just chasing 100% line-of-code coverage. And worse: aren't the specifics of that part of the statement just implementation detail, rather than part of the actual requirement? The feature was "update the order system so that orders requiring a financial transaction require a valid email address". It doesn't say anything about what sort of collections we use to store the order.

Initially I concluded it was just implementation detail and there was no (test) case to answer to here. But it stuck in my craw that we were not testing that part of the conditional logic. I chalked that up to me being pedantic about making sure every line of code has coverage. Remember I generally use TDD when writing my code, so it's just weird to me to have conditional code with no specific tests, because the condition would only ever be written because a feature called for it.

But it kept gnawing at me.

Then the penny dropped (this was a few hours later, I have to admit).

It's not just testing implementation detail. The issue / confusion has arisen because we're not doing TDD.

If I rewind to when we were writing the function, and take a TDD approach, I'd be going "right, what's the first test case here?". The first test case - the reason why want to type in isInstanceOf(this.orderItems, "ServicesCollection") - is because part of the stated requirement is implicit. When the client said "if any items in the order have a cost associated with them…" they were not explicit about "this applies to both orders for services as well as orders for products", because in the business's vernacular the distinction between types of orders isn't one that's generally considered. For a lot of requirements there's just the concept of "orders" (this is why there should be an interface for orders!). However the fully-stated requirement could be made more accurately "if any items in an order for services have a cost associated with them…", followed by "likewise, if any items in an order for products have a cost associated with them…". It's this business information that inform our test cases, which would be better enumerated as:

  • it requires an email address if any items in a services order have an associated cost;
  • it requires an email address if any items in a products order have an associated cost;
  • it does not require an email address if the order is empty;

I had to chase up what the story was if the collection was neither of those types, and it turns out there wasn't a third type that was immune to the email address requirement, it's just - as per the nice clear test case! - the order might be empty, but the object as a whole still needs validation.

There is still "implementation detail" here - the code checking the collection type is obviously the implementation of that part of the business requirement: all code is implementation detail after all. The muddiness here arose because we were backfilling tests, rather than using TDD. Had we used TDD I'd not have been questioning myself when I was deciding on my test cases. The test cases are driven by business requirements; they implementation is driven by the test cases. And, indeed, the test implementation has to busy itself with implementation detail too, because to implement those tests we need to pass-in collections of the type the function is checking for.

As an aside, I was not happy with how busy that if condition was. It seemed to be doing too much to me, and mixing a coupla different things: type checks and a validity check. I re-reasoned the situation and concluded that the validation function has absolutely nothing to do if the email is there and is valid: this passes all cases right from the outset. So I will be recommending we separate that out into its own initial guard clause:

function validateEmail() {
    if (isValid("email", {
    if ((isInstanceOf(this.orderItems, "ServicesCollection") || isInstanceOf(this.orderItems, "ProductsCollection"))) {
        var hasItemsWithCost = this.orderItems.some((item) => item.cost > 0)
        if (hasItemsWithCost.len()) {
            this.addError("Valid email is required")

I think this refactor makes things a bit easier to reason through what's going on. And note that this is a refactor, so we'll do it after we've got the tests in place and green.

I now wonder if this is something I should be looking out for more often? Is a compound if condition that is evaluating "asymmetrical" sub-conditions indicative of a code smell? Hrm… will need to keep an eye on this notion.

Now if only I could get that OrderCollection interface created, the code would be tidier-still, and closer to the business requirements.




PS: I actually doubted myself again whilst writing this, but by the time I had written it all out, I'm happy that the line of thinking and the test cases are legit.