This repro shows how methods defined on a class with the arrow => syntax break the coverage report in WTR in certain situations.

For issues modernweb-dev/web#689 and istanbuljs/v8-to-istanbul#121

To reproduce run npm install then npm test, and open coverage/lcov-report/bad.js.html.

Instead of coverage for the affected file, one will see:

Cannot read property 'decl' of undefined
TypeError: Cannot read property 'decl' of undefined
    at /Users/xxxx/wtr-coverage-repro/node_modules/istanbul-reports/lib/html/annotator.js:89:31
    at Array.forEach ()
    at annotateFunctions (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-reports/lib/html/annotator.js:86:29)
    at annotateSourceCode (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-reports/lib/html/annotator.js:234:9)
    at HtmlReport.onDetail (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-reports/lib/html/index.js:409:33)
    at LcovReport. [as onDetail] (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-reports/lib/lcov/index.js:28:23)
    at Visitor.value (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-lib-report/lib/tree.js:38:38)
    at ReportNode.visit (/Users/xxxx/wtr-coverage-repro/node_modules/istanbul-lib-report/lib/tree.js:88:21)
    at /Users/xxxx/wtr-coverage-repro/node_modules/istanbul-lib-report/lib/tree.js:92:19
    at Array.forEach ()

The suspected cause of this bug is:

1. Each test runs in its own context, and coverage is collected for all files which are loaded in each test run
2. The metadata for functions in a file is stored from the first run that included that file
3. The coverage is combined from all test runs to produce the final coverage numbers
4. Methods authored with the arrow syntax => are only defined at runtime when the constructor is executed, and if in point 2. the constructor in a file isn't executed, the metadata for those methods won't exist.

In this repro:

• 1-test.js loads Good through index.js which also includes Bad.js.
  • Bad.js is executed and so gains some coverage information, but Bad's constructor is never executed, so point 4. above never occurs.
  • The coverage information is collected, but metadata about Bad.method doesn't exist, because that function never did exist in this test run
• 2-test.js runs, and this does execute Bad's constructor and so now there is coverage information for Bad.method
• When the coverage details are generated, and the coverage of Bad.method is collated the metadata for Bad.method does not exist, because the metadata was only stored from the first run of 1-test.js. This is what results in the above stack trace.