✏️ 3.2.3.4 Practice: Café Menu

Goal: Practice using Record and Omit in a new context.

Warm Up

• What does Record<K, V> produce? How is it different from writing an object type by hand?
• What does Omit<T, K> do? Give a concrete example of when it's useful.
• Why would a "create item" function accept a type without an id field?

Code

You're modeling a café menu. Start a fresh TypeScript project (or a new file in your existing one) using these types:

type Category = "coffee" | "tea" | "food" | "other";

type MenuItem = {
  id: number;
  name: string;
  price: number;
  category: Category;
  available: boolean;
};

Write the following:

1. getCategoryCounts(menu: MenuItem[]): Record<Category, number> - returns an object showing how many items are in each category. Every category must be present in the result, even if the count is zero.

2. A NewMenuItem type using Omit that excludes id.

3. createMenuItem - a function that takes a menu and a NewMenuItem, assigns a new id (one higher than the current max), and returns the updated menu with the new item added.

4. filterAvailable - a function that takes a menu and returns only items where available is true.

5. updatePrice - a function that takes a menu, an item id, and a new price. It returns a new menu with that item's price updated and all other items unchanged. If the price is zero or negative, return the menu unchanged or throw an error.

Create a small menu with items in different categories and test each function.

Designing Types From a Description

Once you've finished the café menu exercises, try this type design challenge. Read the description below and write TypeScript types to model it - no functions needed, just the types.

---

You're modeling a community sports league. Here's how it works:

• The league has multiple teams. Each team has a name and a home venue.
• Each team has a roster of players. Each player has a name and a jersey number. Jersey numbers are unique within a team but not across the whole league.
• The league schedules games between two teams. Each game has a date, a home team, and an away team. Games also record the final score for each side - but some scheduled games haven't been played yet, so scores aren't always available.
• Each game has a status: either scheduled, in progress, or completed.

Write the TypeScript types for this system. As you work, consider:

• Which properties are always present, and which might not be?
• Are any properties constrained to a specific set of values?
• How do you represent the relationship between a game and its two teams?

Instructor/Peer Code Review

• Does getCategoryCounts include all four categories in its return value, even ones with zero items?
• Is NewMenuItem derived from MenuItem using Omit rather than written from scratch?
• Does the project compile with no TypeScript errors?
• In the type design section: does the model accurately reflect the description - are the right properties optional, and are constrained values represented as literal union types?