How do I resolve Promise in mocked function?

Question

I'm trying to test method which returns Promise and resolves in another method which I want to mock. I can test it without mocking but I don't want to because of credentials which wouldn't work on another machine.

Here's my Service code ( PollyService.ts ):

@Service()
export class PollyService {
    @Inject()
    private polly: Polly; // aws-sdk Polly
    url: string;

    getSpeech(body: any) {
        return new Promise((resolve) => {
            let voice: string = (voices as any)[body.code];

            let params = {
               exampleParam: example
            };

            this.polly.synthesizeSpeech(params, (error: any, data: any) => {
                if (error) throw error;
                else {
                    resolve(data);
                }
            });
        });
    }
}

Here's test ( app.spec.ts )

describe('PollyService', () => {
    afterEach(() => {
        Container.reset();
    });
    it ('Should return data', async () => {
        const event: any = {
            body: {
                sentence: "potato",
                code: "en"
            }
        };
        let polly = Container.get(Polly); // aws-sdk polly 
        spyOn(polly, 'synthesizeSpeech'); // and here I want to resolve Promise

        await Container.get(PollyService).getSpeech(event.body);
    });
});

When I am mocking that without resolving promise there is error:

Error: Timeout - Async function did not complete within 5000ms (set by jasmine.DEFAULT_TIMEOUT_INTERVAL)

How can i simulate resolving Promise in tests?

Answer 1

If you want to create a promise which is already resolved, you can simply use Promise.resolve(someData)

In your case it should be something like this:

spyOn(polly, 'synthesizeSpeech').and.returnValue(Promise.resolve('mockedData'));

Note: You get the timeout error message because you are neither returning the promise in the end of your function nor make use of the done callback.

either do:

    ...
    return await Container.get(PollyService).getSpeech(event.body);

or

describe('PollyService', () => {
    afterEach(() => {
        Container.reset();
    });
    it ('Should return data', async (done) => { // <- done is the callback
        const event: any = {
            body: {
                sentence: "potato",
                code: "en"
            }
        };
        let polly = Container.get(Polly);
        spyOn(polly, 'synthesizeSpeech');

        await Container.get(PollyService).getSpeech(event.body);
        done(); // <- calling it will tell jasmine your test is over
    });
});

Usually either you choose to return the promise or use the callback. I am not sure on how jasmine mix async methods and the callback in the same method but it should work. More information here

Answer 2

You should extend the spyOn with and.returnValue as follows:

const data = ... // define the data to be returned by synthesizeSpeech
spyOn(polly, 'synthesizeSpeech').and.returnValue(Promise.resolve(data));