Source map remapping has issues on files with no branches or functions

[bcoe]

I noticed this error cropping up for HTML reports:

[LarsDenBakker]

We've been seeing this as well. Any clue on how we could fix this?

[bcoe]

@LarsDenBakker it basically seems to be that the reporting logic is unhappy if there a file contains no branches or functions, is my theory. Interestingly, this only seems to crop up when we're applying source maps, and trying to convert from JavaScript to TypeScript.

What makes this problem so sticky, is that the AST from V8 doesn't necessarily match the AST that TypeScript used to build its source maps.

[bcoe]

To address the issue, someone needs to dig into the output that v8-to-istanbul generates, and figure out what missing fields are causing issues for upstream reporters.

[LarsDenBakker]

I dug into it a while ago, but wasn't able to figure it out at the time :(

[bcoe]

@LarsDenBakker are you still able to reproduce this, I did put a few fixes in place related to this bug a month ago.

[LarsDenBakker]

It looks like we're still on v5, so I'll update and ask if users are still seeing this error.

[mario-aleo]

I've just updated my @web/test-runner to the latest version (0.10.0) and unfortunately, I'm still getting the Cannot read property 'decl' of undefined error :(

Later I'll create a repository with this error and share it with you so you can easily reproduce it :)

[Westbrook]

This definitely intersects with what's going on in here: istanbuljs/istanbuljs#207, as while I get this issue in a handful of places, I can move it around by changing the use of exported fat arrow functions or not in various test contexts...even if those fat arrow methods are leveraged in non-covered files... 🤔

[bcoe]

@mario-aleo @LarsDenBakker if you could provide a minimal reproduction, that would be super helpful 👍

[stephenwade]

@bcoe @mario-aleo @LarsDenBakker I ran into this issue today with @web/test-runner. I was able to narrow it down to one class file and two test files.

If any of these characteristics are changed, the coverage report works properly:

• The class function assigns an anonymous function to an instance property.

export class MyClass {
  fn() {
    this._ = function () {};
  }
}

• There are two test files, both of which import the same class file.
• Only one of the test files calls the class function.

https://github.com/stephenwade/coverage-issue-repro

To reproduce, run npm test and open coverage/lcov-report/my-class.js.html.

[bcoe]

@stephenwade thank you for the reproduction.

[Stuk]

I have a minimal repro repository here: https://github.com/Stuk/wtr-coverage-repro. The details of what I believe causes this issue are listed in the readme.

To summarize, metadata for functions in a file is only collected the first time that file is loaded, however some methods might only be defined when more code executed. Hence the above error when trying to access metadata for a covered function that doesn't always exist.

[bcoe]

@Stuk thank you for the reproduction, not sure when I'll have cycles for OSS in the immediate term, but this will help the fix be quick when I can sit down to look.

[Stuk]

@bcoe I have a fix for this that I was able to find in the https://github.com/modernweb-dev/web repo that I'll be sending a PR for tomorrow. Unfortunately as I was skimming the code I wasn't able to work out how to fix it in this repo.

[LarsDenBakker]

The fix from @Stuk fixes our logic of merging coverage of different test runs on different browsers. I'm not sure if that was the same scenario as originally reported in this issue, it might still occur without merging involved.

[Westbrook]

It is certainly possible, however I can report that the change specifically removed some instances of the following in my project.