NestJs - test on response from interceptor

Question

I have a simple interceptor that deletes a field from response:

import {
    CallHandler,
    ExecutionContext,
    Injectable,
    NestInterceptor,
} from '@nestjs/common';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

export interface Response<T> {
    data: T;
}

@Injectable()
export class Transform<T> implements NestInterceptor<T, Response<T>> {
    intercept(
        context: ExecutionContext,
        next: CallHandler,
    ): Observable<Response<T>> {
        return next.handle().pipe(
            map((response) => {
                delete response?.value?.name;
                return response;
            }),
        );
    }
}

How can I write test case for this? Basically, I wanted to test if 'name' is deleted from response. I wrote the following test case but response is coming as undefined:

  it('should delete `name` from response if present', async () => {
    transformInterceptor = new TransformRepoResponse();
    const context = {
      switchToHttp: () => ({
        getRequest: () => jest.fn(),
      }),
    };
    const next = {
      handle: jest.fn().mockReturnValue({
        pipe: jest.fn(),
      }),
    };
    requestMock.method = 'GET';
    const response = await transformInterceptor.intercept(context, next);
    expect(response.value.name).toBe(undefined);
  });

Jackie McDoniel · Accepted Answer · 2022-10-16 17:30:22Z

2

Interceptors aren't async so you can't await their response without using lastValueFrom() as a wrapper. Also, your next.handle() should return an observable that will be acted on by the .pipe. Something like

it('should delete `name` from response if present', (done) => {
    transformInterceptor = new TransformRepoResponse();
    const context = {
      switchToHttp: () => ({
        getRequest: () => jest.fn(),
      }),
    };
    const next = {
      handle: jest.fn().mockReturnValue(of({
        value: {
          name: 'name-field',
          foo: 'foo-field',
        }
      }),
    };
    requestMock.method = 'GET';
    let errors = []
    transformInterceptor.intercept(context, next).subscribe({
      next: (data) => {
        try {
          expect(data).toEqual({
            value: { foo: 'foo-field' }
          })
        } catch (err) {
          errors.push(err);
        }
      complete: () => done(errors.length ? errors.join(' ') : undefined)
      }
    })
  });

Now you're testing the interceptor as an observable, you only expect one value so using the next of subscribe is fine, and you're telling Jest that the test is finished when the observable completes. You're also persisting any errors that happen by catching them in the next and using them as part of the done call if they exist.

edited Oct 16, 2022 at 17:30

answered Oct 14, 2022 at 14:02

Jackie McDoniel

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6 Comments

Tony Mathew Over a year ago

thanks for your answer. One more question on this, my test case does not cover the branch in the line "delete response?.value?.name;". I wrote two test cases one with no "response", and the other with response, but without "value". But still it says "branch" not covered. How do I write test case to cover all the branches in the line?

Tony Mathew Over a year ago

I found one issue with this solution - That the test case never fails even if: " expect(data).toEqual({ value: { foo: 'foo-field' } }) " fails. Jest marks this test case as passed. Is it because the "expect" statement is given inside "next()"??

Jackie McDoniel Over a year ago

It should report if there's a problem. You can add in error: reject to that object and see if that fixes it

Tony Mathew Over a year ago

Where do I add "error: reject" ? Could you please update your answer?

Jackie McDoniel Over a year ago

Sorry, was thinking about promises with observables. It'd actually be error: done or explicitly error: (err) => done(err)

|

chickenchilli · Accepted Answer · 2022-11-10 01:45:53Z

This worked for me for your example:

import { CallHandler } from '@nestjs/common';
import { lastValueFrom, of } from 'rxjs';
import { createMocks } from 'node-mocks-http';
import { ExecutionContextHost } from '@nestjs/core/helpers/execution-context-host';

describe('Transform Interceptor test', () => {
  let transformInterceptor: Transform<any>;
  beforeEach(() => {
    transformInterceptor = new Transform();
  });

  it('should be defined', () => {
    expect(transformInterceptor).toBeDefined();
  });

  it('should remove the name', async () => {
    const { req, res } = createMocks();
    const testContext = new ExecutionContextHost([req, res]);
    const nextSpy: CallHandler<any> = {
      handle: () =>
        of({ what: 'ever', value: { name: 'Mario', something: 'else' } }),
    };

    await expect(
      lastValueFrom(transformInterceptor.intercept(testContext, nextSpy)),
    ).resolves.toEqual({ what: 'ever', value: { something: 'else' } });
  });
});

I found good hint for this solution here: https://github.com/leosuncin/nest-api-example/blob/master/src/auth/interceptors/token.interceptor.spec.ts

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