gistfile1.js

function* idMaker(prefix){ 
   let idx = 0; 
   while(true){ 
      yield `${prefix}_${idx++}`;    
   } 
}
let users = idMaker("user");
console.log(users.next()) // {"value":"user_0","done":false}
console.log(users.next()) // {"value":"user_1","done":false}

let items = idMaker("item");
console.log(items.next()) // {"value":"item_0","done":false}
console.log(items.next()) // {"value":"item_1","done":false}