Save and Toggle Favorites

Open app.js and build Favorites around the existing from and to state. The finished project stores pair keys such as USD/EUR.

Start with a toggle function:

function toggleFavorite(pair) {
  let favorites = read("fx-favorites");
  favorites = favorites.includes(pair)
    ? favorites.filter(item => item !== pair)
    : [...favorites, pair];

  write("fx-favorites", favorites);
}

For the active pair, the key can come directly from the shared state:

const currentPair = () => `${from}/${to}`;

Test it

Click the Favorite control on Convert and inspect localStorage.

Checkpoint

The same pair should be added once and removed when the control is clicked again.