Adding roots to Jest config

[mucsi96]

test.log

Is your proposal related to a problem?

My problem is that Jest test are running slow. I have tried CRA with TypeScript using npx create-react-app my-app --typescript. After that executing npm test on Windows 10 machine took 35 seconds. I started to investigate why it's so slow. After making a CPU profile I saw huge number of calls to graceful-fs readdir function. After adding a console.log there I discovered that the function was called on every directory inside .git and node_modules folders. See attached log.

Describe the solution you'd like

To try to improve the performance I have added roots: ['<rootDir>/src'], to createJestConfig.js. After that there were no readdir calls apart from src folder. With that the test execution time went down to 20 seconds. This is a 43% speed improvement on my machine.

[mucsi96]

I also wanted to optimize further. Seeing the following CPU profile:

I am wondering why requiring modules all together needs 15 seconds for single component and test. Would be great if someone with more experience on CPU profiling can take a look. Profile attached.

CPU-20190801T141211.zip

[ianschmitz]

You're right - it seems because we've specified rootDir to be the root of the project, based on their docs it will search all the sub directories for tests even though our testMatch doesn't include node_modules.

Good finding! Would you like to create a PR to add roots?