aaron_bot.js

const https = require('https'),
      qs = require('querystring'),
      ACCESS_TOKEN = "ACCESS_TOKEN_FROM_SLACK",
      VERIFICATION_TOKEN = "TOKEN_FROM_SLACK";

// Verify Url - https://api.slack.com/events/url_verification
function verify(data, callback) {
    if (data.token === VERIFICATION_TOKEN) {
        callback(null, data.challenge);
    }
    else {
        callback("verification failed");   
    }
}

// Post message to Slack - https://api.slack.com/methods/chat.postMessage
function process(event, callback) {
    // test the message for a match and not a bot
    if (!event.bot_id && /(clauset|aaron)/ig.test(event.text)) {
        var links = ["http://i.imgur.com/AxOhUDD.jpg", 
                     "http://i.imgur.com/k5YKpGu.jpg", 
                     "http://i.imgur.com/n26ytsd.jpg", 
                     "http://i.imgur.com/t7mzMXa.jpg", 
                     "http://i.imgur.com/7eJGf9O.png", 
                     "http://i.imgur.com/GCPNAav.png",
                     "http://i.imgur.com/lvdKAPM.gif",
                     "http://i.imgur.com/wFgVcma.gif",
                     "http://i.imgur.com/cQZTDt5.jpg",
                     "http://i.imgur.com/qeI2V9C.gif",
                     "http://i.imgur.com/NBo0hEy.png"];
        var text = `<@${event.user}> ` + links[Math.floor(Math.random()*links.length)];
        var message = { 
            token: ACCESS_TOKEN,
            channel: event.channel,
            text: text
            }
        }

        var query = qs.stringify(message); // prepare the querystring
        https.get(`https://slack.com/api/chat.postMessage?${query}`);

    callback(null);
}

// Lambda handler
exports.handler = (data, context, callback) => {
        const done = (err, res) => callback(null, {
        statusCode: err ? '400' : '200',
        body: err ? (err.message || err) : JSON.stringify(res),
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded',
        },
    });
    
    console.log(data.body)
    
    switch (JSON.parse(data.body).type) {
        case "url_verification": 
            verify(JSON.parse(data.body), done)
            break;
        case "event_callback":
            process(JSON.parse(data.body).event, done); 
            break;
        default: 
            done(null);
            break;
    }
};