220127-unique-id-map.js

const preferredStoreItems=[
  {"PreferredStoreName":"Stater Bros","GroceryItemID":3,"Name":"Test","PreferredStoreID":3,"Notes":"Test","CreatedOn":"Apr 30 2021 10:43PM"},
  {"PreferredStoreName":"Ralph's","GroceryItemID":2,"Name":"Baguette","PreferredStoreID":7,"Notes":"","CreatedOn":"Apr 30 2021 10:43PM"},
  {"PreferredStoreName":"Stater Bros","GroceryItemID":1,"Name":"Saltines","PreferredStoreID":3,"Notes":"","CreatedOn":"Apr 30 2021  6:26AM"},
]

const z = preferredStoreItems.reduce((map, item) => {
  map.set(item.PreferredStoreID, item.PreferredStoreName);
  return map;
}, new Map);

console.log(z);
// Map(2) { 3 => 'Stater Bros', 7 => "Ralph's" }

console.log(Object.fromEntries(z));
// { '3': 'Stater Bros', '7': "Ralph's" }