Example for facebook

example-for-facebook1.ts

const smallArray: string[] = []; // initial value IDK?

// unless you need to update the array itself, you can leave with the set.
// If you need to update the array, you can do that close to actions on the set.
const smallSet = new Set<string>(smallArray);

addEventListener('new-array-to-compare', (bigArray: string[]) => {
  for (const value of bigArray) {
    if (smallSet.delete(value)) {
      // value was in smallSet and was deleted

      // here you can remove the item from smallArray if you need.
      // However, as you can see, each deletion would cause an O(n) iteration on the array,
      // so if you can live with the small array being just a Set and not an array, I'd do that.
    }
  }
});

addEventListener('new-item-for-small-array', (newItem: string) => {
  smallSet.add(newItem);
  
  // or
  
  if (!smallSet.has(newItem)) {
    smallSet.add(newItem);
    smallArray.push(newItem); // if you need to update the array as well for some reason
  }
});

example-for-facebook2.ts

const smallArray = [];

addEventListener('new-array-to-compare', (bigArray: string[]) => {
  const bigSet = new Set<string>(bigArray);
  
  // not the prettiest, but iterating over an array and modifying it on the way is pretty ugly.
  // going from the end to the beginning makes it so that the indices will stay valid and you
  // won't have to alter them manually if you end up deleting an index.
  for (let i = smallArray.length - 1; i >= 0; i--) {
    if (bigSet.has(smallArray[i]) {
        smallArray.splice(i, 1)
    }
  }
});

addEventListener('new-item-for-small-array', (newItem: string) => {
  smallArray.push(newItem);
});