How to anticipate an error being thrown by an observable in RxJS

Question

How to anticipate an error being thrown by an observable in RxJS

Within my TypeScript application, there exists a method that produces an rxjs Observable. Under certain conditions, this method may use the throwError function:

import { throwError } from 'rxjs';

// ...

getSomeData(inputValue): Observable<string> {
  if (!inputValue) {
    return throwError('Missing inputValue!');
  }

  // ...
}

What approach should I take to create a test case that covers this specific scenario?

angular typescript rxjs jasmine angular-test

Answer 1

Answer №1

If you want to verify its functionality, you can experiment with RxJS Marble diagram examinations. Below is a method:

const obtainData = (input: string): Observable<string> => {
  if (!input) {
    return throwError('Missing input value!');
  }

  // Example
  return of(input);
};

describe('Error assessment', () => {

  let scheduler: TestScheduler;

  beforeEach(() => {
    scheduler = new TestScheduler((actual, expected) => {
      expect(actual).toEqual(expected);
    });
  });

  it('should generate an error when an invalid value is sent', () => {
    scheduler.run(({ expectObservable }) => {

      const expectedMarbles = '#'; // # denotes an error terminal event

      const result$ = obtainData(''); // An empty string is falsy

      expectObservable(result$).toBe(expectedMarbles, null, 'Missing input value!');
    });
  });

  it('should deliver an input value and then immediately complete', () => {
    scheduler.run(({ expectObservable }) => {

      const expectedMarbles = '(a|)';

      const result$ = obtainData('A valid string');

      expectObservable(result$).toBe(expectedMarbles, { a: 'A valid string' });
    });
  });
});

To learn more about creating these assessments, check out this link.

Answer 2

If you want to verify its functionality, you can experiment with RxJS Marble diagram examinations. Below is a method:

const obtainData = (input: string): Observable<string> => {
  if (!input) {
    return throwError('Missing input value!');
  }

  // Example
  return of(input);
};

describe('Error assessment', () => {

  let scheduler: TestScheduler;

  beforeEach(() => {
    scheduler = new TestScheduler((actual, expected) => {
      expect(actual).toEqual(expected);
    });
  });

  it('should generate an error when an invalid value is sent', () => {
    scheduler.run(({ expectObservable }) => {

      const expectedMarbles = '#'; // # denotes an error terminal event

      const result$ = obtainData(''); // An empty string is falsy

      expectObservable(result$).toBe(expectedMarbles, null, 'Missing input value!');
    });
  });

  it('should deliver an input value and then immediately complete', () => {
    scheduler.run(({ expectObservable }) => {

      const expectedMarbles = '(a|)';

      const result$ = obtainData('A valid string');

      expectObservable(result$).toBe(expectedMarbles, { a: 'A valid string' });
    });
  });
});

To learn more about creating these assessments, check out this link.

Answer 3

Answer №2

My interpretation of the entire scenario might be something along these lines

// Initially, there is a process that emits an Observable
export function executeObservableProcess() {
  return initiateInitialObservable()
  .pipe(
     // Subsequently, you handle the data emitted by the first Observable 
     // and proceed to another operation that will result in emission of another Observable
     // This requires the use of operators like concatMap or switchMap
     // This secondary operation is where errors may occur,
     // and this is where your getSomeData() function comes into play
     switchMap(input => fetchData(input))
  );
}
}

// At some point later, you subscribe to the Observable
executeObservableProcess()
.subscribe(
   results => performActions(results),
   error => handleErrors(error),
   () => finalizeExecution()
)

A typical test for error scenarios could be formulated as shown below

it('test error condition'), done => {
   // Set up the necessary context to trigger an error condition during execution
   .....
   executeObservableProcess()
   .subscribe(
      null, // You are not interested in emitted values
      error => {
        expect(error).to.equal(....);
        done();
      },
      () => {
        // The code here should ideally not run since we anticipate an error condition
        done('Error: The Observable is supposed to throw an error instead of completing successfully');
      }
   )
})

Answer 4

My interpretation of the entire scenario might be something along these lines

// Initially, there is a process that emits an Observable
export function executeObservableProcess() {
  return initiateInitialObservable()
  .pipe(
     // Subsequently, you handle the data emitted by the first Observable 
     // and proceed to another operation that will result in emission of another Observable
     // This requires the use of operators like concatMap or switchMap
     // This secondary operation is where errors may occur,
     // and this is where your getSomeData() function comes into play
     switchMap(input => fetchData(input))
  );
}
}

// At some point later, you subscribe to the Observable
executeObservableProcess()
.subscribe(
   results => performActions(results),
   error => handleErrors(error),
   () => finalizeExecution()
)

A typical test for error scenarios could be formulated as shown below

it('test error condition'), done => {
   // Set up the necessary context to trigger an error condition during execution
   .....
   executeObservableProcess()
   .subscribe(
      null, // You are not interested in emitted values
      error => {
        expect(error).to.equal(....);
        done();
      },
      () => {
        // The code here should ideally not run since we anticipate an error condition
        done('Error: The Observable is supposed to throw an error instead of completing successfully');
      }
   )
})

Answer 5

Answer №3

To ensure that the operator throws an error, you can pipe it as demonstrated below. This assumes that Chai is being used.

import { catchError } from 'rxjs/operators';

it('should throw an error',  done => {
     getSomeData().pipe(catchError((e) => [e])).subscribe(e => {
        expect(e).to.be.an.instanceof(Error);
        done();
      })
})

Source How to test an RxJS operation that throws an error

Answer 6

To ensure that the operator throws an error, you can pipe it as demonstrated below. This assumes that Chai is being used.

import { catchError } from 'rxjs/operators';

it('should throw an error',  done => {
     getSomeData().pipe(catchError((e) => [e])).subscribe(e => {
        expect(e).to.be.an.instanceof(Error);
        done();
      })
})

Source How to test an RxJS operation that throws an error

Answer 7

Answer №4

Furthermore, building on the approach provided by lagoman's solution, you have the option to streamline the process of obtaining the testScheduler.

describe('Error validation', () => {  
  it('should trigger an error when an invalid value is passed', () => {
    getTestScheduler().run(({ expectObservable }) => { // Accessing the getTestScheduler function from jasmine-marbles library

      const expectedMarbles = '#'; // An error terminal event is indicated by #

      const result$ = getSomeData(''); // An empty string evaluates to falsy

      expectObservable(result$).toBe(expectedMarbles, null, 'Missing inputValue!');
    });
  });

  it('should emit an input value and then complete immediately', () => {
    getTestScheduler().run(({ expectObservable }) => {

      const expectedMarbles = '(a|)';

      const result$ = getSomeData('A valid input');

      expectObservable(result$).toBe(expectedMarbles, { a: 'A valid input' });
    });
  });
});

Answer 8

Furthermore, building on the approach provided by lagoman's solution, you have the option to streamline the process of obtaining the testScheduler.

describe('Error validation', () => {  
  it('should trigger an error when an invalid value is passed', () => {
    getTestScheduler().run(({ expectObservable }) => { // Accessing the getTestScheduler function from jasmine-marbles library

      const expectedMarbles = '#'; // An error terminal event is indicated by #

      const result$ = getSomeData(''); // An empty string evaluates to falsy

      expectObservable(result$).toBe(expectedMarbles, null, 'Missing inputValue!');
    });
  });

  it('should emit an input value and then complete immediately', () => {
    getTestScheduler().run(({ expectObservable }) => {

      const expectedMarbles = '(a|)';

      const result$ = getSomeData('A valid input');

      expectObservable(result$).toBe(expectedMarbles, { a: 'A valid input' });
    });
  });
});

Answer 9

Answer №5

After stumbling upon an old question of mine, I managed to solve it using the following method:

it('should trigger an error if ...', (done) => {
  const { service } = setup();

  service.myObs$().subscribe({
    error: (err) => {
      expect(err).toEqual(new Error(`Some error`))
      done();
    },
  });

  // code that causes myObs$ to throw a new error
});

Answer 10

After stumbling upon an old question of mine, I managed to solve it using the following method:

it('should trigger an error if ...', (done) => {
  const { service } = setup();

  service.myObs$().subscribe({
    error: (err) => {
      expect(err).toEqual(new Error(`Some error`))
      done();
    },
  });

  // code that causes myObs$ to throw a new error
});

How to anticipate an error being thrown by an observable in RxJS

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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