index.js

const core = require("@actions/core");
const fs = require("fs");
const Octokit = require("@octokit/rest");

const isLgtmString = (text, triggerWords) => {
  return text.trim().toLowerCase() === triggerWords;
};

// most @actions toolkit packages have async methods
async function run() {
  try {
    // action info
    const repoOwner = process.env.GITHUB_REPOSITORY.split("/")[0];
    const repoName = process.env.GITHUB_REPOSITORY.split("/")[1];
    if (!["issue_comment"].includes(process.env.GITHUB_EVENT_NAME)) return;

    // github client
    const token = core.getInput("token");
    const triggerWord = core.getInput("triggerWord");
    const octokit = new Octokit({
      auth: `token ${token}`,
    });

    // issue info
    const issueInfo = JSON.parse(
      fs.readFileSync(process.env.GITHUB_EVENT_PATH, "utf8")
    );
    if (!isLgtmString(issueInfo.comment.body, triggerWord)) return;
    octokit.issues.createComment({
      owner: repoOwner,
      repo: repoName,
      issue_number: issueInfo.issue.number,
      body:
        "![site-cat](https://user-images.githubusercontent.com/33852683/70984291-8a759d80-20fd-11ea-9e0d-35b559adde8b.jpeg)",
    });
  } catch (error) {
    core.setFailed(error.message);
  }
}

await run();