How to Build a Custom GitHub Action in JavaScript

When a shell step gets too gnarly, a JavaScript action gives you typed inputs, outputs, and the full actions toolkit.

Define action.yml with runs: node20 and main, then use @actions/core in your script to read inputs and write outputs.

Steps

• Create action.yml declaring inputs, outputs, and a node20 runs block.
• Install @actions/core and write index.js that reads inputs and sets outputs.
• Bundle dependencies with ncc so dist/index.js is self-contained.
• Reference the action by path with uses: ./ in a test workflow.

Workflow

# action.yml
name: 'Greet'
inputs:
  who:
    required: true
outputs:
  message:
    description: 'greeting'
runs:
  using: 'node20'
  main: 'dist/index.js'
# index.js
const core = require('@actions/core');
const who = core.getInput('who');
core.setOutput('message', `Hello ${who}`);
# usage
# - uses: ./
#   with: { who: world }

Notes

• Commit the bundled dist/ output, since the runner does not run npm install for the action.
• Latchkey managed runners run these custom-action jobs cheaper and self-heal mid-run.

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