.on('message', message => {
    if (message.content === '!ping') {
        message.channel.send('Pong.');
    } else if (message.content.startsWith('beep')) {
        message.channel.send('Boop.');
    }
});
// The client is the bot itself. 
// You would have defined this at the top of your index.js/app.js
// It would look like 'const client = new Discord.Client()'
// Put that in place for '' and remove the <>