How do you write a custom JavaScript action for GitHub Actions?

A JavaScript action consists of two files at minimum: action.yml (the action metadata) and an entry-point JavaScript file. It runs directly on the runner (no container spin-up), which makes it fast. The @actions/core and @actions/github npm packages provide the toolkit for reading inputs, setting outputs, and interacting with the GitHub API.

action.yml:

name: 'Post PR Comment'
description: 'Posts a comment on the triggering pull request'
inputs:
  message:
    description: 'Comment body'
    required: true
outputs:
  comment-id:
    description: 'ID of the created comment'
runs:
  using: 'node20'
  main: 'dist/index.js'

src/index.js:

const core = require('@actions/core');
const github = require('@actions/github');

async function run() {
  try {
    const message = core.getInput('message', { required: true });
    const token = core.getInput('github-token');
    const octokit = github.getOctokit(token);

    const { context } = github;
    const issue_number = context.payload.pull_request?.number;

    const { data: comment } = await octokit.rest.issues.createComment({
      ...context.repo,
      issue_number,
      body: message,
    });

    core.setOutput('comment-id', comment.id);
  } catch (err) {
    core.setFailed(err.message);
  }
}

run();

Key points:

• Bundle all dependencies into dist/index.js using @vercel/ncc — do not rely on npm install at runtime. Commit the dist/ folder to the action repository.
• core.setFailed() both logs the error message and exits with code 1, marking the step as failed.
• Use using: 'node20' (or node16) in action.yml to declare the Node.js version.
• Test locally with INPUT_MESSAGE="hello" node dist/index.js — inputs are injected as INPUT_<NAME> environment variables.

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