- Stub -

Test double used to simulate an external dependency that sends information into the unit of code.

It lets us control the info received by the unit of code.

Multiple Stubs can exist in a test.

Given that it inserts information into the test WE DO NOT VALIDATE OVER STUBS.

class Person {
  constructor(private _name: string, private _lastname: string) {}

  public get name() {
    return this._name;
  }

  public get lastname() {
    return this._lastname;
  }
}

/**
 * Interface required for reading people from a db
 * */
interface IPersonDBReader {
  readAllPeople(): Promise<Array<Person>>;
}

/**
 * Stub implementation for IPersonDBReader
 * With this stub we can simulate that a DB returned certain people
 */
class StubPersonDBReader implements IPersonDBReader {
  constructor(private people: Array<Person>) {}

  async readAllPeople(): Promise<Person[]> {
    return this.people;
  }
}

/**
 * Real class that we're going to test
 */
class PersonMapper {
  /**
   * Initializes a PersonMapper instance
   * @param _personDBReader data source to read people from
   */
  constructor(private _personDBReader: IPersonDBReader) {
    /**
     * Please notice that we're expecting an IPersonDBReader instance.
     * That is the actual trick that allows us to send a Stub to simulate
     * a real DB with our fake data
     */
  }

  /** Method from PersonMapper that we want to test */
  async mapPersonToFullName(): Promise<Array<String>> {
    const people = await this._personDBReader.readAllPeople();

    return people.map((person: Person) => `${person.name} ${person.lastname}`);
  }
}

/** Test Beginning */

/** Initialize two people for the test */
const freddieMercury = new Person("Freddie", "Mercury");
const brianMay = new Person("Brian", "May");

/** Create a Stub to simulate that two people are read from the DB into the PersonMapper*/
const stubPersonDBReader: IPersonDBReader = new StubPersonDBReader([
  freddieMercury,
  brianMay,
]);

/** Send StubPersonDBReader object to PersonMapper via constructor */
const personMapper: PersonMapper = new PersonMapper(stubPersonDBReader);

/** Call mapPersonToFullName method to test that it's correctly mapping each person to a fullName */
personMapper.mapPersonToFullName().then((fullNames) => {
  const expectedFullNames = ["Freddie Mercury", "Brian May"];
  fullNames.forEach((item, index) => {
    /** Simulate test result validation */
    item === expectedFullNames[index];
  });
});

/** Test End */

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