[Bug] Unable to merge coverage for the same file

[sashuk]

One of my projects started failing with the following message (I use latest https://github.com/ljharb/istanbul-merge that pulls latest https://www.npmjs.com/package/istanbul-lib-coverage)

aleksandr@Alexanders-MacBook-Pro-2 test-portal % npx istanbul-merge --out "coverage/coverage-final-1.json" "./**/coverage/coverage-final.json"
/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:74
            itemLoc.start.line - mapLoc.start.line,
                    ^

TypeError: Cannot read properties of undefined (reading 'start')
    at findNearestContainer (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:74:21)
    at addNearestContainerHits (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:121:23)
    at mergeProp (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:147:25)
    at FileCoverage.merge (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:301:27)
    at CoverageMap.addFileCoverage (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:112:29)
    at /Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:55:18
    at Array.forEach (<anonymous>)
    at CoverageMap.merge (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:54:35)
    at /Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-merge/bin/istanbul-merge:36:6
    at forEachArray (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/for-each/index.js:12:17)

Node.js v18.16.0

It started failing right after https://github.com/istanbuljs/istanbuljs/pull/750/files was published as latest.

After doing some research locally I was able to narrow the problem down to the coverage file, coming from one of the dependencies, particularly node_modules/tsconfig-paths-webpack-plugin/coverage/coverage-final-test.json (istanbul-merge looks into node_modules, there is no easy way to exclude node_modules via glob).

How to reproduce

Please check out https://github.com/sashuk/demo-coverage and run npx istanbul-merge --out "coverage-final-1.json" "./**/coverage-final.json"

aleksandr@Alexanders-MacBook-Pro-2 demo-coverage % npx istanbul-merge --out "coverage-final-1.json" "./**/coverage-final.json"
@@@ other FileCoverage {
  data: {
    path: '/random-file.ts',
    ... some debugging output ...
  }
}
/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:74
            itemLoc.start.line - mapLoc.start.line,
                    ^

TypeError: Cannot read properties of undefined (reading 'start')
    at findNearestContainer (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:74:21)
    at addNearestContainerHits (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:121:23)
    at mergeProp (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:147:25)
    at FileCoverage.merge (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/file-coverage.js:302:27)
    at CoverageMap.addFileCoverage (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:112:29)
    at /Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:55:18
    at Array.forEach (<anonymous>)
    at CoverageMap.merge (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-lib-coverage/lib/coverage-map.js:54:35)
    at /Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/istanbul-merge/bin/istanbul-merge:36:6
    at forEachArray (/Users/aleksandr/.npm/_npx/98709a0041747bf7/node_modules/for-each/index.js:12:17)

Node.js v19.0.0
npm notice 
npm notice New major version of npm available! 8.19.2 -> 10.2.3
npm notice Changelog: https://github.com/npm/cli/releases/tag/v10.2.3
npm notice Run npm install -g npm@10.2.3 to update!
npm notice 

cc @isaacs @SimenB

[isaacs]

It's weird to me that you'd get a range entry that doesn't have a loc property, but since apparently that can happen, adding some defenses to that case seems fine enough.

@SimenB care to take a look at #754 when you get a chance?

[isaacs]

Ohhh, I see. The statementMap is just the location at the top level. It doesn't have a nested loc field, like branchMap and functionMap do. I hadn't hit that in my testing, because it looks like v8-to-istanbul always tracks each entire line separately, so they're always a match and don't have to walk up to find a containing element.

[mattd-tg]

Thanks for making this reproducible issue @sashuk. I am running into the same exact issue. Came here to check and found you already did the leg work 🥇

[sashuk]

Thank you @isaacs for looking into it so fast!

Happy to help @mattd-tg!

[SimenB]

Forgot to do the release 😅

https://github.com/istanbuljs/istanbuljs/releases/tag/istanbul-lib-coverage-v3.2.2