8-declaration-merging.ts

/**
 * (1) "identifiers" (i.e., a variable, class, function, interface)
 * -   can be associated with three things: value, type and namespace
 */

// function foo() {}
// interface bar {}
// namespace baz {
//   export const biz = "hello";
// }

// // how to test for a value
// const x = foo; // foo is in the value position (RHS).

// // how to test for a type
// const y: bar = {}; // bar is in the type position (LHS).

// // how to test for a namespace (hover over baz symbol)
// baz;

// export { foo, bar, baz }; // all are importable/exportable

/**
 * (2) Functions and variables are purely values.
 * -   Their types may only be extracted using type queries
 */
// const xx = 4;
// const yy: typeof xx = 4;

/**
 * (3) Interfaces are purely types
 */
// interface Address {
//   street: string;
// }

// const z = Address; // 🚨 ERROR (fails value test)

/**
 * (4) Classes are both types _and_ values
 */

// class Contact {
//   name: string;
// }

// // passes both the value and type tests

// const contactClass = Contact; // value relates to the factory for creating instances
// const contactInstance: Contact = new Contact(); // interface relates to instances

/**
 * (5) declarations with the same name can be merged, to occupy the same identifier
 */

// class Album {
//   label: Album.AlbumLabel = new Album.AlbumLabel();
// }
// namespace Album {
//   export class AlbumLabel {}
// }
// interface Album {
//   artist: string;
// }

// let al: Album; // type test
// let alValue = Album; // value test

// export { Album }; // 👈 hover over the "Album" -- all three slots filled

/**
 * (6) Namespaces have their own slot, and are also values
 */

// // 💡 they can be merged with classes

// class AddressBook {
//   contacts!: Contact[];
// }
// namespace AddressBook {
//   export class ABContact extends Contact {} // inner class
// }

// const ab = new AddressBook();
// ab.contacts.push(new AddressBook.ABContact());

// // 💡 or functions

// function format(amt: number) {
//   return `${format.currency}${amt.toFixed(2)}`;
// }
// namespace format {
//   export const currency: string = "$ ";
// }

// format(2.314); // $ 2.31
// format.currency; // $