Bad error message in --experimental-test-coverage output when a source maps source module is missing

[jaydenseric]

Version

v22.8.0

Platform

Darwin TRI-N93DLJDY6Y 23.6.0 Darwin Kernel Version 23.6.0: Mon Jul 29 21:14:30 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6000 arm64

Subsystem

No response

What steps will reproduce the bug?

In dist/a.mjs:

console.log("Hi");
export {};
//# sourceMappingURL=a.mjs.map

In dist/a.mjs.map:

{"version":3,"file":"a.mjs","sourceRoot":"","sources":["../src/a.mts"],"names":[],"mappings":"AAAA,OAAO,CAAC,GAAG,CAAC,IAAI,CAAC,CAAC"}

In test.mjs:

import "./dist/a.mjs";

Then, run:

node --experimental-test-coverage --test test.mjs

How often does it reproduce? Is there a required condition?

Every time.

What is the expected behavior? Why is that the expected behavior?

An error message reported after the tests output, explaining that a particular source module at a path is missing. If multiple are missing, then output the list of missing paths.

What do you see instead?

Hi
✔ test.mjs (46.715834ms)
ℹ Warning: Could not report code coverage. TypeError: Cannot read properties of undefined (reading '0')
ℹ tests 1
ℹ suites 0
ℹ pass 1
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 51.063542

Additional information

This scenario comes up very frequently when working in a TypeScript project with source maps enabled, that has src and dist directories. Often you have a build script watching the src directory, and as you work you rename or delete a source module. The result is that old artifacts don't get cleaned in the dist directory, but the next time the tests run on the dist directory the coverage explodes with the error this issue reports.

Note that if multiple test modules run, then the coverage report for everything is obscured by the error message:

ℹ Warning: Could not report code coverage. TypeError: Cannot read properties of undefined (reading '0')

[RedYetiDev]

See also #54444, which resolved a similar error, so maybe they are somewhat related?

[cjihrig]

If someone wants to pick this up, this diff plus the test case provided in the bug report should get you started:

diff --git a/lib/internal/test_runner/coverage.js b/lib/internal/test_runner/coverage.js
index 7ef5702072..4348ea06c3 100644
--- a/lib/internal/test_runner/coverage.js
+++ b/lib/internal/test_runner/coverage.js
@@ -388,8 +388,12 @@ class TestCoverage {
             continue;
           }
 
-          newUrl ??= startEntry?.originalSource;
-          const mappedLines = this.getLines(newUrl);
+          const mappedLines = this.getLines(startEntry.originalSource);
+          if (!mappedLines) {
+            throw new Error('insert better error message here');
+          }
+
+          newUrl ??= startEntry.originalSource;
           const mappedStartOffset = this.entryToOffset(startEntry, mappedLines);
           const mappedEndOffset = this.entryToOffset(endEntry, mappedLines) + 1;
           for (let l = startEntry.originalLine; l <= endEntry.originalLine; l++) {

[RedYetiDev]

throw new Error('insert better error message here');

What even is the issue? Is it:

a particular source module at a path is missing

[cjihrig]

In that snippet, startEntry.originalSource is the name of a file that does not exist for whatever reason. getLines() handles this case, but this code didn't properly check the return value.

[RedYetiDev]

Working on this now.