I’ve spent a good deal of time helping teams adopt automated testing as a part of their workflow; one thing that I’ve often heard from folks who aren’t familiar with the methodology is the following:

“How much time do you estimate for writing unit tests?”

Usually what people are looking for is something like, “I need X hours for the production code, and Y hours for writing tests for this feature.” If you’re familiar with automated testing (and especially TDD), you probably recognize that this is the wrong way to look at it; I’ve struggled with explaining this in the past, but I recently came a across a real example that might be instructive.

We found a bug in some production code that was using a list component; when removing an item from the list, the value argument passed to the event handler was null, but we were looking for value.id. In addition, the POST body for our API call expected the properties to be defined even if they weren’t set, and to have a default value of -1.

The total time to discover, log, reproduce and find the root cause of this bug was on the order of a few hours.

The production code to fix the bug looked like this:

static getCategoryValue(val) { let category = -1; if (val && val.id !== undefined && val.id !== null) { category = val.id; } return category; }

The total time to write that function was on the order of a few minutes.

The test code to check the behavior looked like this:

describe('Category values', () => { it('Returns -1 if the value is null', () => { expect(PoiForm.getCategoryValue(null)).to.equal(-1); }); it('Returns -1 if the value is undefined', () => { expect(PoiForm.getCategoryValue()).to.equal(-1); }); it('Returns -1 if the id is null', () => { const val = {id: null}; expect(PoiForm.getCategoryValue(val)).to.equal(-1); }); it('Returns -1 if the id is undefined', () => { const val = {}; expect(PoiForm.getCategoryValue(val)).to.equal(-1); }); it('Returns the id if it is 0', () => { const val = {id: 0}; expect(PoiForm.getCategoryValue(val)).to.equal(0); }); it('Returns the id if it is 1', () => { const val = {id: 1}; expect(PoiForm.getCategoryValue(val)).to.equal(1); }); })

The total time to write the above tests was on the order of a few seconds.

Obviously these are order of magnitude estimates–your mileage may vary. The point is, the test logic is very simple and quick to write, while the time lost by an uncaught bug is very large–there’s a lot of time and effort spent before we even touch the code. Then we have the build and deploy process, promoting through environments, and the time of our users–they could be losing entire days of productivity. An automated test suite will catch bugs before the code even leaves the developer’s workstation, and keep catching them in future–too often, all of these things are left out of the accounting.

As a general rule, it doesn’t make sense to compare tests and production methods by lines of code, nor to estimate them separately. Ideally we’ll be writing the tests and production code together, using TDD–that’s the goal. Adding a separate line item or estimate for unit testing is backwards, and moves us in the opposite direction.

If you’re asked to estimate the time you spend writing tests, take a step back and try to reframe the conversation–when you’re scoping a piece of work, you really want to be talking in terms of features and defects, rather than specific technical tasks–business language, not technical jargon. Tests are a tool we use to help ensure the features that we build work; trying to split test writing out into a separate task implies that it’s something that could be cut, and that’s a false premise–the only tradeoff is spending a lot more time fixing broken features, which affects the developers, the customer, and the end users.

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