Twilio Function for retrieving cat gifs from The Cat API

catgif.js

exports.handler = function(context, event, callback) {
  var client = context.getTwilioClient();
  
  console.log("Sending initial response")
  client.messages.create({
    to: event.From,
    from: event.To,
    body: "Fetching a cat for you, one sec..."
  }, function(err, res) {
    console.log("Sending a cat!")
    let twiml = new Twilio.twiml.MessagingResponse();
    let message = twiml.message();
    message.body("Here's your cat!");
    message.media("http://thecatapi.com/api/images/get?format=src&type=gif")
    callback(null, twiml);
  })
};

hi i would like to write a twilio function wich send a response base on the incoming message.
lets say, if i send an sms wich says "aaa"
the twiml message will be "answer to aaa"
if i send "bbb"
the twiml message will be 'answer to bbb"
and if i send something else
the twiml message will be "please say aaa or bbb"

i am actually using this function

exports.handler = function(context, event, callback) {
let twiml = new Twilio.twiml.MessagingResponse();
twiml.message("hello you just send a message to your twilio number");
callback(null, twiml);
};

but it will answer this message whatever the input is...

i would like the function to know what is the incoming message body...
can you help me please ?