Discord.js get user by nickname - node.js

Question

I'm having a hard time figuring out
Is is possible to find a user by his or her nickname via the discord.js npm package.
How can I query a user by his or her nickname?
i've tried several things but nothing returns the nickname and I can't quite figure out how their documentation works.
So far I haven't been able to tell how to do it.
I've done as so in my code.
const { Client } = require('discord.js')
const discordClient = new Client()
discordClient.on('message', message => {
if (message.author.bot) return
if (message.content.startsWith('!gpwarn')) {
// The command is something like '!gpwarn thomas'.
// If his nick name is thomas but his username is john it doesn't work.
})

Answer 1

Yes, and it's pretty simple. Do this:
const user = client.users.cache.find(user => user.username == "The user's name");
Reference:
UserManager

Answer 2

To meet your exact requirements you would search the guild members cache for the provided nickname. However, I personally would suggest using either a direct tag or a UserID for this, as multiple people can have the same nickname in the server.
const U = message.guild.members.cache.find(E => E.nickname === 'NICKNAME')
Getting by Tag:
const U = message.mentions.members.first()
Getting by ID:
const U = message.guild.members.cache.find(U => U.id === 'IDHERE')

Answer 3

First you need to identify the nickname argument, you can do this by splitting, slicing, and joining message.content.
Next I recommend you fetch all of the GuildMembers from the guild using GuildMemberManager#fetch(), This way you don't run into the error of a member being uncached.
Handle the promise using async/await and use Collection#find() to return the member.
const { Client } = require('discord.js')
const discordClient = new Client()
discordClient.on('message', async message => {
if (message.author.bot) return
if (message.content.startsWith('!gpwarn')) {
// Returns all of the text after !gpwarn
const query = message.content.split(' ').slice(1).join(' ').toLowerCase()
const members = await message.guild.members.fetch()
const memberToWarn = members.find(m => m.nickname.toLowerCase() === query)
if (!memberToWarn) // No member found error
console.log(memberToWarn) // [GuildMember]
}
})