# - Mock -

[Test double](https://blog.fiftywords.co/test-double) used to simulate an [external dependency](https://blog.fiftywords.co/external-dependency-exit-point) that **receives** information sent by the [unit of code](https://blog.fiftywords.co/unit-of-code).

It allows to validate that information..

It can only exist **ONE MOCK** at most per test, given that it simulates an [exit point](https://blog.fiftywords.co/exit-point) and only one is verified per [unit test](https://blog.fiftywords.co/unit-test)

```ts
/** Interface tha defines an external dependency */
interface ILogger {
  logInfo(info: string);
}

/** Real implementation for ILogger interface */
class RealLogger implements ILogger {
  public logInfo(info: string) {
    console.log(info);
  }
}

/** Mock implementation to verify
 * Calculator.add is logging properly */
class MockLogger implements ILogger {
  /** Private field to store last logged info */
  private receivedInfoToLog: string;

  /** Method defined by ILogger interface */
  public logInfo(info: string) {
    /** Assign received info to this.receivedInfoToLog
     * so we can later verify that Calculator.add sent
     * the right info
     */
    this.receivedInfoToLog = info;
  }

  /** Give access to validate latest logged info */
  public get loggedInfo(): string {
    return this.receivedInfoToLog;
  }
}

class Calculator {
  /** Costructor receiving an ILogger implementation */
  constructor(private logger: ILogger) {}

  public add(summand: number, otherSummand: number): number {
    /** Uses received ILogger implementation to log info
     * related to addition */
    this.logger.logInfo(`Will add ${summand} and ${otherSummand}`);
    return summand + otherSummand;
  }
}

/** Test Beginning */

/** Initializes a MockLogger object to validate
 * Calculator is properly logging data */
const mockLogger: MockLogger = new MockLogger();
/** Send MockLogger object to Calculator via constructor */
const calculator: Calculator = new Calculator(mockLogger);
/** Call add method to test that it's correctly logging the info */
calculator.add(4, 5);

/** Verify that the logged info is as expected */
mockLogger.loggedInfo === "Will add 4 and 5";

/** Test End */
```
[🇨🇴 Versión en Español](https://blog.cincuentapalabras.co/mock)