[j1elo]

Agree with both. Recently, youtube channel ThePrimeagen was talking about this, and coincidentally put up a very silly but clarifying example, luckily he also posted to Twitter so here it is just for fun [1]:

  function foo(num: number): number {
    const a = [1];
    let sum = 0;
    for (let i = 0; i < num; ++i) {
      sum += a[i];
    }
    return sum;
  }

  test("foo", () => {
    expect(foo(1)).toEqual(1);
  });

  100% test coverage
  100% still bugged af

[1]: https://nitter.net/ThePrimeagen/status/1639250735505235975

[randomdata]

> 100% still bugged af

That isn't necessarily a problem. The primary purpose of your tests is to document the API. As long as someone is able to determine what the function is for, and how it is meant to be used, along with other intents and information the author needs to convey to users of the API, the goal is met.

However, I don't see how the given test actually helps document anything, so it is not a good example in the slightest. 100% coverage doesn't indicate that you did a good job, but if there are code paths not touched then you know you screwed up your documenting big time.

[imiric]

That's a contrived example that misses the point, and I think poisons the discussion.

Code coverage, and especially _line_ coverage, doesn't tell you anything about the quality of the tests. It's just a basic metric that tells you how much of the code is being, well, covered, by at least one test. Most projects don't even achieve 100% line coverage, and, like most of you here, I agree that reaching that is often not very productive.

But, if in addition to line coverage you also keep track of branch coverage, and use other types of tests (fuzz, contract, behavior, performance, etc.), then your chances of catching a bug during development are much higher. After all, let's not forget that catching bugs early, or ideally not even committing them, is the entire purpose of testing. All of this work takes a lot of effort and discipline, of course, which is why most projects don't bother with it. But, again, you can't argue that the ROI is not worth it, when projects like SQLite are one of the most stable programs in existence, in a large part due to their very high testing practices. This doesn't mean that this is not worth pursuing, or that the entire practice is wrong for some reason.

Arguing that coverage is not a meaningful metric is a reflection of laziness IMO.