articles.001.008.key-extractor-in-component.tsx

export function Select<T>({ items, value, idFromValue }: SelectProps<T>) {
  // selectedId will be a string/number value that we can use to identify the selected item
  const selectedId = value === undefined ? undefined : idFromValue(value);
  
  return <div>
    {items.map(item => {
      const id = idFromValue(item);
      
      // selected will now be true for values with matching IDs
      const selected = id === selectedId;
      
      return <div key={id}/>;
    })}
  </div>;
}