Parallelizing Jest in GitHub Actions

ci.yml

name: CI
on: [push]
jobs:
  setup:
    runs-on: ubuntu-latest
    outputs:
      test-chunks: ${{ steps['set-test-chunks'].outputs['test-chunks'] }}
      test-chunk-ids: ${{ steps['set-test-chunk-ids'].outputs['test-chunk-ids'] }}
    steps:
      - uses: actions/checkout@v2
      - run: npm install
      - id: set-test-chunks
        name: Set Chunks
        run: echo "::set-output name=test-chunks::$(npx jest --listTests --json | jq -cM '[_nwise(length / 2 | ceil)]')"
      - id: set-test-chunk-ids
        name: Set Chunk IDs
        run: echo "::set-output name=test-chunk-ids::$(echo $CHUNKS | jq -cM 'to_entries | map(.key)')"
        env:
          CHUNKS: ${{ steps['set-test-chunks'].outputs['test-chunks'] }}
  test:
    runs-on: ubuntu-latest
    name: test (chunk ${{ matrix.chunk }})
    needs:
      - setup
    strategy:
      matrix:
        chunk: ${{ fromJson(needs.setup.outputs['test-chunk-ids']) }}
    steps:
      - uses: actions/checkout@v2
      - run: npm install
      - name: jest
        run: echo $CHUNKS | jq '.[${{ matrix.chunk }}] | .[] | @text' | xargs npx jest
        env:
          CHUNKS: ${{ needs.setup.outputs['test-chunks'] }}

i got this error when using the above github actions

Error reading JToken from JsonReader. Path '', line 0, position 0.,.github/workflows/chunks.yml (Line: 75, Col: 16): Unexpected type of value '', expected type: Sequence.

any idea how to resolve this?

Do you have to specify the length manually, or is there a way to set up that automatically? For instance, for every 100 tests, create a new test chunk