How can you create a dynamic matrix where the list of values is determined by a previous job?
Select an answer to reveal the explanation.
Short Explanation and Infographic
fromJSON() converts a JSON string output from a prior job into an actual array that the matrix strategy can iterate over.
Full explanation below image
![Educational infographic 1024x768 showing dynamic matrix pattern. Job A (setup): runs script, outputs matrix='["16","18","20"]' to GITHUBOUTPUT. Arrow to Job B with matrix: node: ${{ fromJSON(needs.setup.outputs.matrix) }}. Result: three matrix jobs for node 16, 18, 20. fromJSON() function converting string to array shown with type labels. Real-world use case: discovered test environments.](/media/github/gh-200/GH-200-q073-infographic.webp)
Full Explanation
Dynamic matrices are achieved by: (1) Having a 'setup' job output a JSON array string via $GITHUB_OUTPUT (e.g., 'matrix=["node16","node18","node20"]'); (2) The downstream job with 'needs: setup' uses fromJSON() in its matrix: 'matrix: node: ${{ fromJSON(needs.setup.outputs.matrix) }}'. This enables matrices computed from external sources (API calls, file contents, git tags). Artifact-based approaches don't integrate with the matrix syntax. GitHub API calls can't be made inside YAML matrix blocks. Dynamic matrices are fully supported.