I tried on googling but all of the errors was "is not a function", that's because i'm using discord.js V12? I can't find it on google, there is less question about this one. This was my code.
var serverArray = client.guilds.array();
for(i = 0; i < serverArray.length; i++) {
console.log("Server ID: " + serverArray[i].id);
}