When a person joins a server in discord, discord, by default, sends a message saying something like "I hope you brought pizza, user!" I want to detect these messages using discord.js, any help would be appreciated.
You can use the Message#type property to determine if it is a welcome message. The type will be GUILD_MEMBER_JOIN in that case.
if (message.type === 'GUILD_MEMBER_JOIN') {
message.channel.send('Welcome to the server!')
}