Tutorial: Combining Jest Test Coverage Reports with IstanbulJS in GitHub CI

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Tutorial: Combining Jest Test Coverage Reports with IstanbulJS in GitHub CI

Sometimes developers using Jest testing framework need to unify the test reports that they have scattered across different repository projects. Unfortunately, there is no automated tool that can do that operation all at once for now, but can be achieved by scripting the operation with the command-line interface of the InstanbulJS coverage report tool - nyc.

This tutorial shows how to set up a GitHub Actions workflow that collects coverage reports from two different repositories, merges them, and outputs the final coverage report in a third main repository.

Step 1: Setup the Repositories

Ensure each of the two project repositories is set up for Jest testing and coverage reporting. Then, create a third repository where the merged coverage report will be stored.

For example:

First repository includes a simple function to capitalize a string while second repository has a simple function to add two numbers. Third Main repository has a GitHub CI workflow file that generates test reports in each of two repositories with simple functions. After generating reports, CI workflow merges and store the unified coverage report back in the Main repository.

Step 2: Create GitHub Personal Access Token

For the GitHub Actions workflow in the third repository to access and clone the other two repositories, you need a Personal Access Token (PAT) with the appropriate permissions (e.g., repo scope).

  1. Go to your GitHub settings.

  2. Click on "Developer settings" → "Personal access tokens".

  3. Generate a new token with the necessary permissions.

  4. Save this token securely; you will not be able to see it again.

Step 3: Add the Personal Access Token to the Third Repository

In the third repository (where the final report will be generated):

  1. Go to the repository settings.

  2. Navigate to "Secrets" → "Actions".

  3. Create a new secret (e.g., COVERAGE_TOKEN) and paste the Personal Access Token you generated.

Step 4: Create a GitHub Actions Workflow in the Third Repository

In your third repository, create a new GitHub Actions workflow file (e.g., .github/workflows/merge-coverage.yml):

name: Merge Coverage Reports

on: [push]

    runs-on: ubuntu-latest

    - name: Checkout Main Repository
      uses: actions/checkout@v2

    - name: Setup Node.js
      uses: actions/setup-node@v2
        node-version: '14'

    - name: Install NYC
      run: npm install -g nyc

    - name: Checkout Repo 1
      uses: actions/checkout@v2
        repository: 'username/repo1'
        token: ${{ secrets.COVERAGE_TOKEN }}
        path: 'repo1'

    - name: Checkout Repo 2
      uses: actions/checkout@v2
        repository: 'username/repo2'
        token: ${{ secrets.COVERAGE_TOKEN }}
        path: 'repo2'

    - name: Generate Coverage Report for Repo 1
      run: |
        cd repo1
        npm install
        npm run test -- --coverage
        cp -r coverage ../coverage-repo1
      working-directory: ${{ github.workspace }}

    - name: Generate Coverage Report for Repo 2
      run: |
        cd repo2
        npm install
        npm run test -- --coverage
        cp -r coverage ../coverage-repo2
      working-directory: ${{ github.workspace }}

    - name: Merge Coverage Reports
      run: |
        nyc merge coverage-repo1 coverage1.json
        nyc merge coverage-repo2 coverage2.json
        nyc report --reporter lcov --temp-directory . --report-dir final-coverage-report
      working-directory: ${{ github.workspace }}

    - name: Upload Coverage to Artifacts
      uses: actions/upload-artifact@v2
        name: merged-coverage-report
        path: final-coverage-report

Step 5: Push and Trigger the Workflow

Push this workflow file to your main repository. This will trigger the GitHub Actions workflow, which will:

  1. Checkout each project repository.

  2. Run tests and generate coverage reports.

  3. Merge the coverage reports using nyc.

  4. Save the merged coverage report in the main repository's final-coverage-report directory.

Step 6: Download and Review the Final Report

After the workflow completes, you can download the merged coverage report from the 'Artifacts' section in the GitHub Actions run.


  • Replace username/repo1 and username/repo2 with your actual repository names.

  • Ensure that your test scripts in package.json of repo1 and repo2 are correctly set up to generate coverage reports.

  • This workflow assumes that the projects are Node.js-based and are using Jest for testing. Adjust the workflow according to your specific tech stack and requirements.

  • Always keep your PAT secure and use it cautiously as it provides access to your GitHub repositories.


  1. Jest: How to merge coverage reports from different jest test runs