Fool's Gold: Code Coverage

Ryan James Spencer

If you are unfamiliar with code coverage, the idea is simple: you write accompanying tests to code and a code coverage tool produces reports of lines covered by tests and the percentage of that coverage to all lines of code. The hope is that a higher coverage with tests means you'll have a 'correct' system. I have even heard of some establishments initiating quotas on required coverage per lines of new code being introduced. "If it doesn't have tests it doesn't exist" is the usual argument for this requirement; code without tests is potentially problematic code, but tests are also untested chunks of code in our codebase. For example, consider this bit of React code:

test('Breadcrumb renders', () => {
  expect(() => {

What is this testing exactly? Literally any other test, even one without the toThrow expectation, would mark this as a failure on an exception being thrown. This will light up code coverage though. People learn to cheat the system, or please the percentage going up, and focus less on the guarantees that tests are providing them. This does not help us deliver better products to end users.

Code coverage percentage is a useless metric. There is no way to know what percentage of code written is code your end users actually care about when that percentage is derived from synthetic traffic. You may write a continent of code, but only a thousandth of that code may actually be hit by users. If you are using code coverage to tell you that you have greater than zero percent code coverage than you have a code organization issue or there is the possibility that many or all of your tests are false positives.

Here's a different approach: instrument your application to track invocations of code paths. You can do with this tracing, structured logging, profiling replayed traffic, etc. The technique employed doesn't matter but what does is determining what is valuable and what is dead. Regardless of collecting metrics, you will always need to consider this from a human perspective.

Detractors may argue that code that doesn't immediately show usage should not be hastily deleted. They are partly right. If things are early on at your company, doing eyeball statistics may be fair but eyeball statistics is not real statistics. Practicing some basic statistical understanding is always in order for any kind of analysis. It may take time to reach a statistically significant result and whether one is reached should, more often than not, drive your decisions. As noted we need to exercise judgement despite what the numbers may tell us. Perhaps the piece of code in question is a critical piece of error handling that is rarely executed, for example, or maybe the code is serving a particularly infrequent, but high-paying, user base.

I'd like to stress that I am not saying that testing is a pointless errand. What I am saying is that code coverage is fool's gold. Tests take more effort to create because developers must check that a test is actually testing what you think it's testing. Testing is a part of one's confidence level in what they ship, and when we assert important properties of a system are upheld you boost that confidence level. Don't buy into the idea that coverage is going to lead to a correct system by default. Be vigilant with you tests and instrument your application.