Istanbul Coverage + Svelte: Incorrectly Identifying an If / Else Block

[kgladfelder]

Describe the bug

In a svelte.kit application, while outputting data in an each block, any time the iterator is accessed, Istanbul is reporting that I never go into an if block that doesn't exist. Also, in the coverage report it shows line 97 as partially covered and the HTML shows it as fully covered.

Reproduction

https://github.com/kgladfelder/vitest-issue-repro

System Info

System:
    OS: Windows 10 10.0.22000
    CPU: (16) x64 Intel(R) Core(TM) i7-10875H CPU @ 2.30GHz
    Memory: 7.88 GB / 15.86 GB
  npmPackages:
    vite: ^3.0.7 => 3.0.7
    vitest: ^0.22.0 => 0.22.0

Used Package Manager

npm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
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• The provided reproduction is a minimal reproducible example of the bug.

[AriPerkkio]

I don't think there is anything we can do on Vitest's codebase. Changes in Svelte compiler could fix this.

Looking at @kgladfelder's repro we can see that Svelte compiler is adding helper-functions which end up getting instrumented. I think these helper functions are included in source maps so that debugging tools can stop on breakpoints. But when instrumenting code, we end up collecting coverage from these unnecessary if-blocks. When the transpiled code is mapped back to sources, we can see covered and uncovered lines on unexpected lines+columns.

This is why }</span> is seen in coverage report as uncovered:

Look at these lines+columns in sources:

12 |   <span>{result.gameName}</span>
                              ^^^^^

In the transpiled code those map into this single character here:

225 |   if (dirty & /*searchResults*/ 1 && t0$_value$ !== (t0$_value$ = /*result*/ ctx[2].gameName + "")) set_data_dev$(t0$, t0$_value$);
                                                                                                 ^

Now looking at the instrumented transpiled code. A branch counter is added here. When code is executed it is never reached.
This will mark those original lines in source code as uncovered.

4980 |      if ((cov_1a56qtdo1c().b[7][0]++, dirty &
4981 |      /*searchResults*/
4982 |      1) && (cov_1a56qtdo1c().b[7][1]++, t0$_value$ !== (t0$_value$ =
4983 |      /*result*/
4984 |      ctx[2].gameName + ""))) {
4985 |        cov_1a56qtdo1c().b[6][0]++;
     |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Branch counter!
4986 |        cov_1a56qtdo1c().s[92]++;
4987 |        set_data_dev$(t0$, t0$_value$);
4988 |      } else {

In the same repro repo there are also some 100% correctly working source maps. Here is one example:

16 |  {#if !searched}
            ^^^^^^^^
17 |   <h6>Please search for a game.</h6>
18 |  {:else}

44 | if (!/*searched*/ ctx[1]) return create_if_block$;
                       ^^^

4672 |    if (!
4673 |    /*searched*/
4674 |    ctx[1]) {
4675 |      cov_1a56qtdo1c().b[0][0]++;
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Branch counter!
4676 |      cov_1a56qtdo1c().s[6]++;
4677 |      return create_if_block$;
4678 |    } else {

I'm 99% sure I've seen some code transformer tool adding /* istanbul ignore */ comments around their helper functions. I think it was in Babel's codebase but can't find it anymore.

A possible solution here would be if Svelte compiler excluded its helper functions from instrumenters.

[AriPerkkio]

Upstream issue open sveltejs/svelte#7824.

[AriPerkkio]

This is fixed on Svelte 5.