Jest Empty Coverage Bug

This repo is to demonstarte a bug of jest empty coverage.

Steps

1. Use yarn to install dependencies

yarn install

2. Generate the coverage report.

yarn cover:jest

3. Check the file coverage/coverage-final.json. The coverage data for BrandTable.jsx is incorrect. Some value is null and -1.

    "statementMap": {
      "0": {
        "start": { "line": 1, "column": 0 },
        "end": { "line": null, "column": -1 }
      },
      "1": {
        "start": { "line": 4, "column": 22 },
        "end": { "line": null, "column": -1 }
      },
      "2": {
        "start": { "line": 9, "column": 4 },
        "end": { "line": null, "column": -1 }
      },
      "3": {
        "start": { "line": 13, "column": 0 },
        "end": { "line": null, "column": -1 }
      }
    },

4. Add a unit test to BrandTable.jsx by renaming the src/__tests__/BrandTable-spec to src/__tests__/BrandTable-spec.js. Now the report data for BrandTable.jsx is not generate through empty report.

5. Generate coverage report

yarn cover:jest

6. Check the file coverage/coverage-final.json. Now the coverage data for BrandTable.jsx is correct.

    "statementMap": {
      "0": {
        "start": { "line": 5, "column": 4 },
        "end": { "line": 5, "column": 16 }
      },
      "1": {
        "start": { "line": 9, "column": 4 },
        "end": { "line": 9, "column": 33 }
      },
      "2": {
        "start": { "line": 13, "column": 0 },
        "end": { "line": 13, "column": 37 }
      }
    },