Split Tests in Jest

Splits test files in Jest and ensures all parallel jobs finish work at a similar time.

Before

| | Job | Time |
| --------------------------- | :-: | :--------: |
| =========================== | 1 | 46 minutes |
| ======== | 2 | 13 minutes |
| ============ | 3 | 19 minutes |

After

| | Job | Time |
| ------------- | :-: | :--------: |
| ============= | 1 | 26 minutes |
| ============= | 2 | 26 minutes |
| ============= | 3 | 26 minutes |

Setup

Install @split-tests/jest package:

npm install --save-dev @split-tests/jest
yarn add -D @split-tests/jest

In jest.config.js:

``js
module.exports = {
/ ... /

// IMPORTANT: set the test sequencer
testSequencer: require.resolve("@split-tests/jest"),

// report the run times of the tests, use jest-unit reporter
reporters: [
"default",
[
"jest-junit",
{
outputDirectory: "unit",
addFileAttribute: true,
},
],
],

globals: {
"split-tests": {
// collect the times
junit: "/unit/junit.xml",
},
},
};
`

Usage

Jobs are calculated at run time:

Using environment variables

$ JEST_JOBS_TOTAL=3 JEST_JOBS_INDEX=0 jest

- JEST_JOBS_TOTAL - total number of jobs
- JEST_JOBS_INDEX - index number of the job (starts with 0)

Using cli arguments

$ jest --jobsTotal=3 --jobsIndex=0

- --jobsTotal - total number of jobs
- --jobsIndex - index number of the job (starts with 0)

Custom test reporter

It is possible to read reports not only from a junit file but from any source.

Imagine a this scenario, Jest + jest-junit but saved in CircleCI or in some database.

`js
module.exports = {
/ your jest config /

globals: {
"split-tests": {
async reader(tests) {
const reports = await fetchReports(tests.map((t) => t.path));

return reports;
},
},
},
};
`

The reader is a function that has to return Array<{path: string; time: number}>` (or as a Promise) and its first argument is a list of tests to be executed in Jest.