readme.md

Partitioning CRA tests

1. Install dependencies

$ yarn add --dev @craco/craco @jest/test-sequencer

2. Create jest-sequencer.js and jest.config.js.

3. Alter package.json to use the new config file.

"test": "react-scripts test",
"test": "yarn jest --config=jest.config.js",

4. Add the envs JEST_TOTAL_PARTITIONS, JEST_PARTITION to the CI config file

  test:
    needs: install
    name: Test
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        jest-total-partitions: [4]
        jest-partition: [1, 2, 3, 4]

    steps:
      - name: Check out Git repository
        uses: actions/checkout@v2

      - name: Set up Node.js
        uses: actions/setup-node@v1
        with:
          node-version: 14.17.4

      - name: test
        run: yarn test
        env:
          JEST_TOTAL_PARTITIONS: ${{ matrix.jest-total-partitions }}
          JEST_PARTITION: ${{ matrix.jest-partition }}

jest-sequencer.js

const Sequencer = require('@jest/test-sequencer').default;

class CustomSequencer extends Sequencer {
  sort(tests) {
    const totalPartitionsEnv = process.env.JEST_TOTAL_PARTITIONS;
    const partitionEnv = process.env.JEST_PARTITION;

    if (totalPartitionsEnv && partitionEnv) {
      const totalPartitions = parseInt(totalPartitionsEnv, 10);
      const partition = parseInt(partitionEnv, 10);

      if (Number.isNaN(totalPartitions) || totalPartitions < 1) {
        throw new Error(
          `Invalid total partitions. Expected a number bigger than 1. Got: ${totalPartitionsEnv}.`
        );
      }

      if (
        Number.isNaN(partition) ||
        partition < 1 ||
        partition > totalPartitions
      ) {
        throw new Error(
          `Invalid partition. Expected a number between 1 and ${totalPartitions}. Got: ${partitionEnv}.`
        );
      }

      console.log(`Running partition ${partition} of ${totalPartitions}`);

      return Array.from(tests)
        .sort((a, b) => (a.path < b.path ? -1 : 1))
        .filter((_, i) => i % totalPartitions === partition - 1);
    }

    return tests;
  }
}

module.exports = CustomSequencer;

jest.config.js

const { createJestConfig } = require('@craco/craco');

const cracoConfig = require('./craco.config.js');
const base = createJestConfig(cracoConfig);
const jestConfig = {
  ...base,
  testSequencer: "./jest-sequencer.js"
};

module.exports = jestConfig;