Slap User Discord.js

client.on('message', message => {
    console.log(message.content);
    if (message.content.startsWith(prefix + 'slap')) {
        if (message.mentions.users.first()) {
            message.channel.send(message.author.username + " Slapped " + message.mentions.users.first().username + " With A Palm of Justice!! ")
        } else {
            message.channel.send("Please Mention Who To Slap")
        }
    }
});
Undefined