Map is a JavaScript collection for storing key/value pairs when ordinary objects are too restrictive. Keys and values may use arbitrary types, while TypeScript generic arguments can restrict the collection to a specific key and value type. section 2.2.6 section 4.4.2
Indexing to-do items by numeric ID
A typed collection can declare private itemMap = new Map<number, TodoItem>();. Each todo item's numeric id becomes its key: itemMap.set(item.id, item). A lookup uses itemMap.get(id), while [...itemMap.values()] produces an array that can be filtered before display. The generic declaration lets the compiler reject nonnumeric keys or values that are not TodoItem objects. section 2.2.6
Mapping product names to objects
A generic collection can use a product's name as its key: Map<string, T>. Adding an item calls this.items.set(newItem.name, newItem), and get(name) returns the matching object. The constrained type parameter ensures that T has the name property required by the map operations.
