How to get a code coverage report with react-create-app? - reactjs

Question

If I run
npm test --coverage
the tests pass, but the coverage is not run.
When I change package.json to have
react-scripts test --coverage
Then, when I do npm test and a it runs the tests but not the coverage
The tests run along with the coverage report but the coverage shows all zeroes for coverage
PASS src/components/Header/SubHeader.test.js
The <SubHeader /> component
✓ should render (4ms)
✓ should render with a goback button (1ms)
✓ should render
----------|----------|----------|----------|----------|-------------------|
File | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files | 0 | 0 | 0 | 0 | |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests: 3 passed, 3 total
Snapshots: 3 passed, 3 total
Time: 1.077s
Ran all test suites.
Finally, I realized that I can do
npm run test .
Question: Why don't the other methods work?

Answer 1

After revisiting this question i figured out that if you run this you will trigger coverage command and get all results.
SOLUTION 1
npm run test -- --coverage .
When you are passing arguments to npm script, you need to add -- before adding arguments like --coverage. This is just how npm works. See this answer for passing arguments to npm
SOLUTION 2
yarn test --coverage .
EDIT:
I asked a question regarding . being passed down the command and answer is pretty much simple. . is passed because it is a positional parameter, in contrast to --coverage that is option for npm test command: Why is . passed as argument to npm without -- delimiter?
---- Problem analysis ----
PROBLEM 1
npm test --coverage you are not passing --coverage argument to test script.
In the other case when you edit script to be:
react-scripts test --coverage and run npm test here you actually add argument --coverage, but in script itself, not from npm command
PROBLEM 2
When you do npm run test . you are passing . that means you want to include all files in this command.
For some reason . is passed to the npm run test script but not --coverage.
On the other side yarn passes all arguments without --.