This setup helps to run Jest tests on Gitlab in parallel

.gitlab-ci.yml

stages:
  - prepare
  - build
  - test
  - post_test

unit tests:
  timeout: 20 minutes
  stage: test
  variables:
    CI_NODE_INDEX: $CI_NODE_INDEX
    CI_NODE_TOTAL: $CI_NODE_TOTAL
  parallel: 5
  script:
    - npm run test:ci
  artifacts:
    name: test-reports/
    paths:
      - test-reports/
  cache:
    key: jest
    paths:
      - .tmp/cache/jest/

# =========================
# post test
# =========================
test report:
  stage: post_test
  needs:
    - unit tests
  script:
    - npm run test:ci:test-report
  artifacts:
    paths:
      - test-reports/
    reports:
      junit: test-reports/test-results.xml

coverage:
  stage: post_test
  needs:
    - unit tests
  script:
    - npm run test:ci:coverage-report
  artifacts:
    name: test-coverage
    paths:
      - test-reports/
    reports:
      coverage_report:
        coverage_format: cobertura
        path: test-reports/cobertura-coverage.xml
  coverage: '/Total Coverage: (\d+\.\d+\%)/'

custom-coverage-reporter.js

const { create } = require('istanbul-reports')
const { createCoverageMap } = require('istanbul-lib-coverage')
const { createContext } = require('istanbul-lib-report')
const { resolve } = require('path')
const { sync } = require('glob')
const { GLOBAL_THRESHOLDS } = require('../../jest.config')

const coverageMap = createCoverageMap()

const REPORTS_FOLDER = 'test-reports'
const coverageDir = resolve(__dirname, `../../${REPORTS_FOLDER}`)
const reportFiles = sync(`${coverageDir}/*/coverage/coverage-final.json`)

const COVERAGE_TYPES = ['lines', 'statements', 'functions', 'branches']

/* eslint-disable no-console */

// Normalize coverage report generated by jest that has additional "data" key
// https://github.com/facebook/jest/issues/2418#issuecomment-423806659
const normalizeReport = report => {
    const normalizedReport = Object.assign({}, report)
    Object.entries(normalizedReport).forEach(([k, v]) => {
        if (v.data) normalizedReport[k] = v.data
    })
    return normalizedReport
}

/**
 * prepare unit test coverage files and merge to single test context to build a report
 */
reportFiles
    .map(reportFile => {
        console.log('Found report file: ' + reportFile)
        return require(reportFile)
    })
    .map(normalizeReport)
    .forEach(report => coverageMap.merge(report))
const context = createContext({
    coverageMap: coverageMap,
    dir: REPORTS_FOLDER,
})

/**
 * create and output a Cobertura report for MR coverage visualization
 * https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html
 */
create('cobertura', {}).execute(context)
console.log(
    `Cobertura coverage report generated and outputted to ${coverageDir}`
)

/**
 * create coverage summary and check for met threshold gates, job should
 * fail if any defined thresholds are not met
 */
// create json coverage summary, which also prints summary to terminal
create('json-summary', {}).execute(context)
const coverageSummary = require(`${coverageDir}/coverage-summary.json`)

/**
 * print results to terminal for easy visibility of coverage summary, text-summary
 * will print out directly a coverage summary of each type, and then print a custom
 * total coverage line for use to display total % in merge requests
 */
create('text-summary', {}).execute(context)
const totalSum = COVERAGE_TYPES.map(
    type => coverageSummary.total[type].pct
).reduce((total, percent) => total + percent, 0)
const avgCoverage = totalSum / COVERAGE_TYPES.length
console.debug(`
========= Total Coverage ==============
Total Coverage: ${avgCoverage.toFixed(2)}%
=======================================
`)

/**
 * use the JSON summary report to do checks against coverage thresholds that
 * cannot be used in parallelization. check each type and determine if threshold
 * is met, otherwise fail this job
 */
const checkCoverageAgainstThresholds = coverageSummary => {
    const total = coverageSummary?.total || {}

    return COVERAGE_TYPES.map(type => {
        // If the threshold is a number use it, otherwise lookup the threshold type
        var threshold = GLOBAL_THRESHOLDS[type]

        // Check for no threshold
        if (!threshold) {
            return {
                type,
                skipped: true,
                failed: false,
            }
        }

        const value = total[type]?.pct
        return {
            type,
            required: threshold,
            value,
            failed: value < threshold,
        }
    })
}

const coverageThresholds = checkCoverageAgainstThresholds(coverageSummary)
const failedCoverageTypes = coverageThresholds.filter(type => type.failed)
if (failedCoverageTypes.length > 0) {
    failedCoverageTypes.forEach(({ type, value, required }) => {
        console.error(
            '\x1b[31m%s\x1b[0m',
            `Global coverage threshold for ${type} (${required}%) not met: ${value}%`
        )
    })
    console.log(`❌ Coverage thresholds have not been met`)
    process.exit(1)
}

console.log(`✅ Coverage thresholds met`)
process.exit(0)

jest-tests-custom-sequencer.js

/**
 * Custom Jest test sequencer, taken from Gitlab's codebase. Used to separate
 * and run Jest test suites in parallel based on available job nodes.
 * Structure is based on the default sequencer from Jest.
 */
const Sequencer = require('@jest/test-sequencer').default

/* eslint-disable no-console */
class ParallelCiSequencer extends Sequencer {
    constructor() {
        super()
        this.ciNodeIndex = Number(process.env.CI_NODE_INDEX || '1')
        this.ciNodeTotal = Number(process.env.CI_NODE_TOTAL || '1')
    }

    sort(tests) {
        const sortedTests = this.sortByPath(tests)
        const testsForThisRunner = this.distributeAcrossCINodes(sortedTests)

        console.log(`CI_NODE_INDEX: ${this.ciNodeIndex}`)
        console.log(`CI_NODE_TOTAL: ${this.ciNodeTotal}`)
        console.log(`Total number of tests: ${tests.length}`)
        console.log(
            `Total number of tests for this runner: ${testsForThisRunner.length}`
        )

        return testsForThisRunner
    }

    sortByPath(tests) {
        return tests.sort((test1, test2) => {
            if (test1.path < test2.path) {
                return -1
            }
            if (test1.path > test2.path) {
                return 1
            }
            return 0
        })
    }

    distributeAcrossCINodes(tests) {
        return tests.filter((test, index) => {
            return index % this.ciNodeTotal === this.ciNodeIndex - 1
        })
    }
}

module.exports = ParallelCiSequencer

jest.config.js

const aliasList = require('./internals/webpack/aliasList')

const IS_PARALLEL_TESTS = process.env.CI_NODE_INDEX && process.env.CI_NODE_TOTAL
const GLOBAL_THRESHOLDS = {
    branches: 76,
    functions: 81,
    statements: 84,
    lines: 84,
}

module.exports = {
    /**
     * for pipeline test steps, include junit report for Gitlab report artifacts
     * and custom coverage reporter for regex of coverage %'s
     */
    reporters: [
        'default',
        [
            'jest-junit',
            {
                outputDirectory: IS_PARALLEL_TESTS
                    ? `<rootDir>/test-reports/test-${process.env.CI_NODE_INDEX}-${process.env.CI_NODE_TOTAL}`
                    : '<rootDir>/test-reports',
                outputName: 'test-results.xml',
            },
        ],
    ],
    maxWorkers: 4,
    collectCoverageFrom: ['src/**/*.js'],
    coveragePathIgnorePatterns: [
        '/node_modules/(?!intl-messageformat|intl-messageformat-parser).+\\.js$',
        '/testing/',
    ],
    /**
     * for pipeline test steps, include cobertura reporter for Gitlab coverage report
     * and json-summary for custom coverage reporter
     */
    coverageReporters: [
        'json',
        'lcov',
        ['cobertura', { file: 'coverage-results.xml' }],
        // `json-summary`'s `coverage-summary.json` output needed for custom coverage reporter
        'json-summary',
        /**
         * `text-summary` removes giant coverage matrix from terminal print-out,
         * replace with "text" if you wish to see this locally
         */
        'text-summary',
    ],
    // remove coverage thresholds for cicd parallel tests as they will be incomplete suites
    coverageThreshold: IS_PARALLEL_TESTS
        ? {}
        : {
              global: GLOBAL_THRESHOLDS,
          },
    coverageDirectory: IS_PARALLEL_TESTS
        ? `<rootDir>/test-reports/test-${process.env.CI_NODE_INDEX}-${process.env.CI_NODE_TOTAL}/coverage`
        : '<rootDir>/test-reports/coverage',
    cacheDirectory: '<rootDir>/.tmp/cache/jest',
    // export for use in custom coverage threshold gates when parallel test
    GLOBAL_THRESHOLDS,
}