JAVASCRIPT

discord.js get user by id

client.users.cache.find(user => user.id === 'USER-ID')

discord.js get username

message.author.username

check user by id discord js

if(!message.author.id ==`329023088517971969` || !message.member.hasPermission(['MANAGE_GUILD'])) return message.channel.send(`You don't have the permissions to do this!`)
<code here>

discord.js get user by username

client.users.cache.find(user => user.username == 'USERMAME')