Read JSON file in GitHub Actions

I want to read a JSON file and use a property in a string in a GitHub Actions YAML file. How do I do this? (I want the version of the package.json.)

Use the built-in fromJson(value) (see here: https://docs.github.com/en/actions/learn-github-actions/expressions#fromjson)

Reading a file depends on the shell you're using. Here's an example for sh:

name: Test linux job
on:
  push

jobs:
  testJob:
    name: Test
    runs-on: ubuntu-latest
    steps:
      - id: set_var
        run: |
          content=`cat ./path/to/package.json`
          # the following lines are only required for multi line json
          content="${content//'%'/'%25'}"
          content="${content//$'\n'/'%0A'}"
          content="${content//$'\r'/'%0D'}"
          # end of optional handling for multi line json
          echo "::set-output name=packageJson::$content"
      - run: |
          echo "${{fromJson(steps.set_var.outputs.packageJson).version}}"

Multi line JSON handling as per https://github.com/orgs/community/discussions/26288#discussioncomment-3251220.

GitHub issue about set-env / set-output multi line handling: https://github.com/actions/toolkit/issues/403