index.js

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

async function run(){
    try {

      // requires GitHub Token to allow PR updates
      const ghToken = core.getInput('token', {required: true})

      // custom data to render besides pull_request context
      const customInput = getCustomInput();

      // replacement for {{ opening tag
      const openingTag = getOpeningMustacheTag();

      // replacement for }} closing tag
      const closingTag = getClosingMustacheTag();

      // Get the JSON webhook payload for the event that triggered the workflow
      const payload = JSON.stringify(github.context.payload, undefined, 2)
      //core.info(`The event payload: ${payload}`);

      // Show PR details
      const pr = github.context.payload.pull_request;
      core.info(`Pull request body: ${pr.body}`);
      core.info(`Pull request title: ${pr.title}`);
      if(customInput){
        Object.entries(customInput).forEach(([key, value]) => {
          core.info(`Custom Property ${openingTag} custom.${key} ${closingTag} will render ${value}`);
        })
      }


      const viewData = {...pr, custom:customInput}

      mustache.tags = [openingTag,closingTag]
      // update PR body
      const octokit = github.getOctokit(ghToken);
      const request = {
        owner: github.context.repo.owner,
        repo: github.context.repo.repo,
        pull_number: github.context.payload.pull_request.number,
        body: mustache.render(pr.body, viewData),
        title: mustache.render(pr.title, viewData),
      }
      core.info(`update request: ${request}`);
      const response = await octokit.rest.pulls.update(request);

      core.info(`Response: ${response.status}`);
      if (response.status !== 200) {
        core.error(`Updating the pull request has failed: ${response.text}`);
      }

    } catch (error) {
      core.setFailed(error.message);
    }
}

function getOpeningMustacheTag(){
  const customTag = core.getInput("openingTag", {required: false})
  return customTag ? customTag : "{{";
}

function getClosingMustacheTag(){
  const customTag = core.getInput("closingTag", {required: false})
  return customTag ? customTag : "}}";
}

function getCustomInput(){
  const customInput = core.getInput("customInput", {
    required: false,
  });

  try {
    return parseArray(customInput).reduce((customInputObject, keyValueString) => {
      if(!keyValueString) return customInputObject;
      const indexOfSeparator = keyValueString.indexOf(":");
      const key = keyValueString.substring(0, indexOfSeparator);
      const value = keyValueString.substring(indexOfSeparator + 1);
      return {...customInputObject, [key]:value }
    },{})
  }catch(error){
    core.error(`Failed to parse custom input: ${error}`);
  }

  return null;
}

const parseArray = (val) => {
  const array = val.split('\n')
  const filtered = array.filter((n) => n)
  return filtered.map((n) => n.trim())
}

run();