The next important step is that 
user-input should do different things 
depending on the game_mode.
- In pre-start mode, it changes to running mode.
 
- In running mode, it makes the bird jump.
 
- In game-over mode, it resets the game and puts it back in running mode. (Why not back to pre-start mode? Well, we're saying "click to play again", so if they click, we need to start playing straight away without going through the initial welcome-screen again)
 
function Got_Player_Input(MyEvent) {
   switch (game_mode) {
      case 'prestart': {
                        game_mode = 'running';
                        break;
                        } 
      case 'running': {
                        bird.velocity_y = jump_amount;
                        break;
                        } 
      case 'over': {
                    reset_game();
                    game_mode = 'running';
                    break;
                    } 
       } // switch
   MyEvent.preventDefault();
   }
Hopefully you can see that the 
English description pretty much matches the code one-for-one.
The reset_game function doesn't exist yet. That's next.