Behn's Ramblings | Speeding up Jest on GitHub Actions

I've been working on a project at work recently which is seeing us migrate from CircleCI to GitHub Actions. We use jest to test our frontend code, and to match the speed of our CircleCI setup, we needed to parallelize our tests. For this, we're using a combination of GitHub's Larger Runners (Specifically an 8-core runner, using Ubuntu 20.04) and Jest's built-in sharding capabilities. In this post, I'll show you how you can do the same, but using GitHub's standard runners.

Note: You'll see that I use ${{ github.event.pull_request.head.sha || github.sha }} here - this is because the github.sha works in a way you probably don't expect. The github.sha is the sha of the most recent merge commit to the branch, not the HEAD of the PR branch. This means that if you're running on a PR, you'll need to use the PR's head SHA to get the correct results.

The setup

Firstly, you'll need to configure your GitHub Actions workflow to use a matrix strategy. This will allow you to run multiple jobs in parallel, and will be the basis for the sharding.

name: Jest Tests
on:
  push:
    branches:
      - main
  pull_request:
    types: [opened, reopened, synchronize]
 
# Cancel the workflow if a new push is made before the current one finishes, but only on branches other than main
concurrency:
  group: ${{ github.ref }}-${{ github.workflow }}
  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
jobs:
  test:
    runs-on: ubuntu-latest # This can be changed to a larger runner
    strategy:
      matrix:
        shard: [1, 2, 3, 4] # This can be increased or decreased based on your needs
    env:
      # Increase the memory limit for node to 16GB (Increase this to match your runner size)
      NODE_OPTIONS: --max_old_space_size=16384
    steps:
      - name: Checkout code
        uses: actions/checkout@v4
      - name: Get CPU count
        run: echo "CPUS=$(nproc 2>/dev/null || echo 2)" >> $GITHUB_ENV
      - name: Setup node & yarn
        uses: actions/setup-node@v4
        with:
          node-version: 'lts/Iron' # Latest LTS (v20) at time of writing
          cache: 'yarn'
          cache-dependency-path: 'yarn.lock'
      - name: Run Jest
        run: yarn test --ci --maxWorkers=$CPUS --shard=${{ matrix.shard }}/${{ strategy.job-total }}
      - name: Move test results
        run: |
          mkdir -p /tmp/test-results
          mv junit.xml /tmp/test-results/junit-${{ matrix.shard }}.xml
      - name: Upload test results # This is optional, but useful for tracking test results in a later step
        uses: actions/upload-artifact@v4
        with: # This will upload the test results to the artifacts for use in the reporting step
          name: test-results-${{ github.event.pull_request.head.sha || github.sha }}-${{ matrix.shard }}
          path: /tmp/test-results

After we run the tests, we'll want to report their status. For this example, I'm using dorny/test-reporter to report the test results back to GitHub Actions. It's worth going and reading the docs for this action, as it has a tonne of options for different test reporters, reporting modes, etc.

...
  comment:
    runs-on: ubuntu-latest
    needs: test
    if: success() || failure() # Only run this job if the test job has passed or failed - not if it was cancelled
    steps:
      - name: Checkout code
        uses: actions/checkout@v4
      - name: Download test results
        uses: actions/download-artifact@v4
        with:
          # This will download all the test results from the previous job
          pattern: test-results-${{ github.event.pull_request.head.sha || github.sha }}-*
      - name: Publish Test Results
        uses: dorny/test-reporter@v1
        with:
          name: Jest Tests
          path: /tmp/test-results/*.xml
          reporter: jest-junit

Behn Hayhoe

Speeding up Jest on GitHub Actions

The setup