Add Labels to PRs with a Typescript GitHub Action

GitHub Actions are a powerful tool to automate your workflow. They can be used to run tests, deploy your code, publish a package, and much more.

The cool thing is, there's a GitHub Actions Marketplace where you can find a lot of actions created by... the community.

But what if you can't find the action you need? You can create your own and publish it there!

How to use this tutorial

The full tutorial (all chapters at once) is also available as a video, which you can find here:

Chapter 4: Use case - Adding a label to new Pull Requests

We learned the basics, we have our MVP ready to be expanded so let's do it! We'll create a new action that will automatically run every time a new Pull Request is created and it will add a needs-review label to it.

Update the action code

In order to add a label to a Pull Request, we need to use the GitHub REST API. We can use the Octokit client to make the API calls. Luckily for us there's an official package we can use.

npm install @actions/github

Then we can import the Octokit client and use it to add the label to the Pull Request. Here's the full code.

import { getInput, setFailed } from '@actions/core';
import { context, getOctokit } from '@actions/github';

async function run() {
  const token = getInput('gh-token');
  const label = getInput('label');

  const octokit = getOctokit(token);
  const pullRequest = context.payload.pull_request;

  try {
    if (!pullRequest) {
      throw new Error('This action can only be run on Pull Requests');
    }

    await octokit.rest.issues.addLabels({
      owner: context.repo.owner,
      repo: context.repo.repo,
      issue_number: pullRequest.number,
      labels: [label],
    });
  } catch (error) {
    setFailed((error as Error)?.message ?? 'Unknown error');
  }
}

run();

What happened here?

1. We read the gh-token and label inputs from the workflow. The token is a personal access token that we need to create and pass to the action. More on that later. The label is the name of the label we want to add to the Pull Request.
2. We create an instance of the Octokit client using the token.
3. We get the Pull Request number from the context.payload.pull_request object. This object is only available when the action is triggered by a Pull Request event.
4. We call the octokit.rest.issues.addLabels method to add the label to the Pull Request.

In case something fails, we catch the error and we set the workflow as failed with the setFailed function, also coming from the @actions/core package.

Update the action definition

We also need to update the action definition to add the new inputs.

inputs:
  gh-token:
    description: 'The GitHub token for authentication.'
    required: true
  label:
    description: 'The label to be applied to the pull request.'
    required: true

Update the workflow

Finally, we need to update the workflow to run when PRs are opened or reopened:

on:
  pull_request:
    types: [opened, reopened]

...and to pass the new inputs to the action.

- uses: ./
  with:
    gh-token: ${{ secrets.GITHUB_TOKEN }}
    label: 'needs-review'

We're passing the GITHUB_TOKEN secret to the action. It usually does not require you any extra action, as GitHub automatically creates it for you.

In case you get an error that says "Resource not accessible by integration", you need to make sure your token on that repository has write access. You can do that by going to the Settings tab > Actions > General and scroll down to the "Workflow permissions" section.

Setting it to "Read and write permissions" will be enough.

Run the action

We can now run the action again and see the label is added to the Pull Request.

However, let's not forget to run npm run build first, then we can commit and push the changes. The reason is that we told GitHub that the action is located in the dist folder, so we need to make sure that folder is up to date.

npm run build
git add .
git commit -m "Add label to new Pull Requests"
git push

Cool! We can now create a new branch, change a file and open a Pull Request. We'll see the action running and adding the label to the Pull Request, aaaand...

Error: Cannot find module '@actions/core'

What happened here? If you look at your file in dist/index.js, you'll see that the @actions/core package is not there. That's because we didn't tell TypeScript to include it in the build.

A good way to include packages and condense everything in one file is with the tool @vercel/ncc. Let's install it.

npm install @vercel/ncc

Then we can update our build script in package.json to use it.

 "build": "tsc && ncc build lib/index.js"

We also need to change outDir in tsconfig.json to lib.

"outDir": "lib"

You might also want to double-check that lib is in your .gitignore file but dist is not. This is our desired behavior as lib will only have our code in javascript, but dist will have the whole action.

Run the action (2.0)

We can now run npm run build again, then commit and push the changes.

Create a new branch, change a file and open a Pull Request. This time, you'll see the action running and adding the label to the Pull Request.

Closing

And that was it for today! if you have any question or suggestion, feel free to add a comment :)

See you in the next chapter!

Thanks for reading this article, I hope you found it interesting!

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Leonardo MontiniFollow

Awarded GitHub Star in 2023 ⭐️ I talk about Open Source, GitHub, and Web Development. I also run a YouTube channel called DevLeonardo, see you there!