Welcome to today's lesson, where we will tackle a common challenge in software engineering: introducing complex features while preserving backward compatibility. We'll use a Potluck Dinner organization system as our backdrop, embarking on a fascinating journey of TypeScript programming, step-by-step analysis, and strategic thinking. Are you ready? Let's begin our adventure!

Initially, our Potluck Dinner organization system enables us to add and remove participants and manage their respective dishes for each round. There are three critical methods with type specifications:

• addParticipant(memberId: string): boolean: This method adds a participant. If a participant with the given memberId already exists, it won't create a new one but will return false. Otherwise, it will add the member and return true.
• removeParticipant(memberId: string): boolean: This method removes a participant with the given memberId. If the participant exists, the system will remove them and return true. Otherwise, it will return false. When removing a participant, you need to remove their dish if they brought one.
• addDish(memberId: string, dishName: string): boolean: This method enables each participant to add their dishes for every round. If a participant has already added a dish for this round or if the memberId isn't valid, the method will return false. Otherwise, it will add the dish for the respective participant's round and return true.

Let's write our TypeScript code, which implements the functions as per our initial state:

In this code, we use the following Data Structures:

• participants: Set<string>: This ensures that participant IDs remain unique. Using a Set simplifies membership checks (.has) and additions (.add) with $O(1)$ complexity.
• dishes: Record<string, string>: Maps each participant's ID to their dish. This structure allows efficient key-value lookups and updates.

Our Potluck Dinner organization system is currently simple but practical. To make it even more exciting, we're going to introduce a "Dish of the Day" feature. This feature will enable participants to vote, and the dish receiving the most votes will be declared the "Dish of the Day."

To add this feature, we will define two new methods with type annotations:

• vote(memberId: string, voteId: string): boolean: This method will allow a participant to cast a vote for a dish. Each participant can cast a vote only once per round. If a participant tries to vote again or if the memberId isn't valid, it should return false.
• dishOfTheDay(): string | null: This method will calculate and return the "Dish of the Day" based on the votes received. If multiple dishes tie for the highest number of votes, the dish brought by the participant who joined first will take precedence. If there are no votes, the function returns null.

Next, we will extend our existing Potluck class to accommodate these new features with appropriate type specifications:

Firstly, we altered our participants data structure into an Object, where the key is the participant's ID and the value is the time of joining (we use the time of adding to the system as the join time).

We also added a votes object to store the votes, where the key is the participant ID and the value is the participant ID for whom they have cast the vote.

In the vote function, we simply check whether the memberId exists and whether the vote hasn't been cast before. hasOwnProperty is used to check if a specific property exists directly on the object (not inherited though the prototype chain). By checking votes.hasOwnProperty(memberId), we ensure the memberId exists as a key in the participants object, so only valid participants (those who have been added) can vote. !this.votes.hasOwnProperty(memberId) checks if the memberId is not already present in the votes object and ensures that each participant can vote only once per round.

In the dishOfTheDay function, we start by ensuring we perform the calculation only if there are votes. Afterward, we calculate the count of votes for each dish, determine the dish or dishes that have received the maximum votes, and if there's a tie, we select the dish brought by the participant who joined the earliest. Finally, we return the selected "Dish of the Day."

The countVotes method converts an array of votes (votes) into an object (count). For each dish in the array, checks if count[dish] exists. If not, initialize it to 0. Increments count[dish] by 1. For the sample input ['dish1', 'dish2', 'dish1'], the output is { 'dish1': 2, 'dish2': 1 }.

As we introduce the two advanced functionalities, vote and dishOfTheDay, our existing system needs to be adeptly updated to ensure seamless integration while maintaining backward compatibility. Here's how we've refined the previous methods:

The addParticipant method initially just added a participant to a set. With the introduction of the "Dish of the Day" feature, we now store the participant ID along with their join time in an object. This timestamp is crucial for determining the precedence in case of a tie in votes.

This adjustment ensures that every new participant is tracked with a join time, allowing us to leverage this information for the "Dish of the Day" feature.

The original removeParticipant method did not consider the impact of removing a participant who might have already voted or added a dish. To maintain the integrity of our voting and dish tracking, we now clean up all related entries across dishes and votes objects when a participant is removed.

The addDish method's logic remained mostly unchanged since it primarily interacts with the dishes object; however, ensuring that the method checks for the validity of the memberId against the updated participants' storage is essential.

• Participants Data Structure Change: By transitioning from a set to an object for storing participant details, we've inherently kept the unique identification trait while adding the functionality for storing join times. This change is backward compatible as it requires no modification in how participant IDs are provided or handled externally.

• Method Signature Consistency: All method signatures (addParticipant, removeParticipant, addDish) remain unchanged. This deliberate decision ensures that existing integrations with our system interface seamlessly continue without modifications from client code.

Combining all our steps, this is the final implementation of our Potluck class with all the required methods:

Bravo! You've successfully completed the task of introducing complex features while preserving backward compatibility. This skill is invaluable in real-life software engineering scenarios, where existing codebases can be thousands or even millions of lines long, and breaking changes can be catastrophic. Strengthen this ability with even more practice and explore similar challenges. I'll see you in the next lesson! Happy coding!