I have been working on a project using a mono-repo layout to build well scoped modules. Of course some unit tests are available to ensure good quality and maintainability over time. However, one thing didn’t work properly: coverage.
Seems there is an issue with the underlying libs (ts-jest, Istanbul, jest, ….) to make the coverage feature in Jest works properly. There is an open issue on Github.
After some research and tries …. I have managed to find a weird combo that make the coverage works.
To make it short, Jest has introduced a while back a way to split configuration per project inside a repository. This feature allows to trick how Jest and other libs work together.
The projects key property is an array of object, each object represents a project within the repository. It is possible to reuse almost all main configurations for each project. It is a neat feature if you need specific rules per folder (or project…)!
So, in my use cases, I have created 2 projects one for packages and one for examples.
{
"projects": [{
"displayName": {
"name": "Packages",
"color": "magenta"
},
"preset": "ts-jest",
"testMatch": [
"<rootDir>/packages/*/src/*.test.ts",
"<rootDir>/packages/*/src/**/*.test.ts"
]
}, {
"displayName": {
"name": "Examples",
"color": "blue"
},
"preset": "ts-jest",
"testMatch": [
"<rootDir>/examples/api-contentful/**/*.test.ts"
]
}]
}
The important part of the trick is to make sure the collectCoverageFrom is well configured with no base root defined. Something like this:
"collectCoverageFrom": [
"**/*.{ts,tsx}",
"!**/*.test.{ts,tsx}",
"!**/dist/**"
]
With projects + collectCoverageFrom you can now have a proper working coverage for ts-jest + mono-repository. The code might used different paths to resolve what files are being used or not using the projects settings