Index.razor

@code
{
    public class Friend
    {
        public string Name { get; set; }
        public int ShoeSize { get; set; }
    }

    private async Task GetFriends()
    {
        var friends =
            await JsRuntime.InvokeAsync<List<Friend>>("getFriends");
    }

    private async Task PutFriend()
    {
          var friend = new Friend
          {
              Name = "Nicolas",
              ShoeSize = 8
          };
          var key = await JsRuntime.InvokeAsync<string>("putFriend", friend);
    }
}