Check if user ID exists in Discord server

If you have the Guild object, you can use the Guild.member() method.

let guild = client.guilds.get('guild ID here'),
  USER_ID = '123123123';

if (guild.member(USER_ID)) {
  // there is a GuildMember with that ID
}

This is very similar to Way to check if a channel exists and the solution should be identical. Essentially, you need to get the guild collection, and then use the discord.js 'Collection.exists' helper function to check if the element (user id) exists in the collection (channel user list).

If in doubt, always check the documentation. :)

https://discord.js.org/#/docs/main/stable/class/Collection?scrollTo=exists

EDIT : Upon further reading, I noticed that 'Collection.exists' is deprecated. The documentation suggests using 'Collection.has' in it's place.

You might find this to be helpful, provided you have an array of server member IDs and the ID of the member you are looking for: How do I check if an array includes an object in JavaScript?

You can use Array#includes to check to see if an array contains a specified object, or in this case your member ID.

guild.member(USER_ID) - is a legacy syntax

If the above doesn't work for you, chanses are you are using discord.js v12 so that you'll have to do this:

let guild = client.guilds.get('guild ID here'),
  USER_ID = '123123123';

if (guild.member.fetch(USER_ID)) {
  // there is a GuildMember with that ID
}

Note that fetch() is an async function which returns a 'promise'. The code above is enough for checking whether the user is a member of a guild but if you wish to read the return value of guild.member.fetch(USER_ID) then you'll have to do the following

guild.members.fetch(usrID) 
 .then((data) => console.log(data));