Sporadic UnhandledPromiseRejectionWarning surfacing while utilizing sinon

Question

Sporadic UnhandledPromiseRejectionWarning surfacing while utilizing sinon

Upon inspection, it appears that the objects failApiClient and explicitFailApiClient should be of the same type. When logging them, they seem to have identical outputs:

console.log(failApiClient) // { getObjects: [Function: getObjects] }

console.log(explicitFailApiClient) // { getObjects: [Function: getObjects] }

After reading this question, I learned how to handle this situation correctly. However, the reason why the generated failApiClient triggers a warning while explicitFailApiClient does not is still unclear.

I have simplified this scenario to its core for replication purposes and to showcase functional alternatives:

import * as sinon from 'sinon';
import 'source-map-support/register';

class LocalObject {
}

const fakeObject = new LocalObject();

const getFakeApi = (result: Promise<LocalObject[]>) = ({getObjects: () => result});

const successObjectClient = getFakeApi(Promise.resolve([fakeObject]));

// Both lines below should behave similarly, yet one causes a test error
const failApiClient = getFakeApi(Promise.reject(new Error()));

const explicitFailApiClient = {
  getObjects(): Promise<LocalObject[]> {
    return Promise.reject(new Error());
  }
};

describe('successApiClient', () => {
  before(() => {
    sinon.spy(successObjectClient, 'getObjects');
  });

  it('does not generate a warning', async () => {
    // do nothing
  });

});

describe('failApiClient', () => {
  before(() => {
    sinon.spy(failApiClient, 'getObjects');
  });

  it('should not produce a warning', async () => {
    // do nothing
  });
});

describe('explicitFailApiClient', () => {
  before(() => {
    sinon.spy(explicitFailApiClient, 'getObjects');
  });

  it('does not trigger a warning', async () => {
    // do nothing
  });
});

The outcome of running

~/...> tsc && npm test

:

> <a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="0e67607a6b7c606f62236f7e674e3f203e203e">[email protected]</a> test /Users/./Projects/./node/internal-api
> grunt test

Running "test" task

Running "env:dev" (env) task

Running "simplemocha:unit" (simplemocha) task


(node:72101) UnhandledPromiseRejectionWarning: Error
    at Object.<anonymous> (/Users/./Projects/./node/internal-api/src/test/unit/models/mvp.test.ts:21:57)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (/Users/./Projects/./node/internal-api/node_modules/coffee-script/lib/coffee-script/register.js:45:36)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
    at /Users/./Projects/./node/internal-api/node_modules/mocha/lib/mocha.js:222:27
    at Array.forEach (<anonymous>)
    at Mocha.loadFiles (/Users/./Projects/./node/internal-api/node_modules/mocha/lib/mocha.js:219:14)
    at Mocha.run (/Users/./Projects/./node/internal-api/node_modules/mocha/lib/mocha.js:487:10)
    […]

(node:72101) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise […]
  successApiClient
    ✓ does not have a warning

  failApiClient
    ✓ should not have a warning

(node:72101) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously […]

  explicitFailApiClient
    ✓ does not have a warning


  3 passing (14ms)


Done.

javascript typescript promise mocha.js sinon

Answer 1

Answer №1

The two scenarios are not the same.

When looking at the code snippet, it's clear that in the first case JS immediately executes the statement Promise.reject as an argument, resulting in the early warning UnhandledPromiseRejectionWarning.

const failApiClient = getFakeApi(Promise.reject(new Error()));

In contrast,

const explicitFailApiClient = {
  getObjects(): Promise<LocalObject[]> {
    return Promise.reject(new Error());
  }
};

The Promise.reject in this second scenario is evaluated only when

explicitFailApiClient.getObjects()

is invoked.

A Different Approach

To address this issue differently, another solution can be implemented using Sinon's resolves and rejects.

const getFakeApi = {getObjects: (result) => result};
const getFakeApiStub = sinon.stub(getFakeApi, 'getObjects');

describe('successApiClient', () => {
  before(() => {
    getFakeApiStub.resolves([fakeObject]); // success and resolves
  });

  it('avoids triggering a warning', async () => {
    // no action is required
  });

});

describe('failApiClient', () => {
  before(() => {
    getFakeApiStub.rejects(new Error()); // force it to fail
  });

  it('should not raise a warning', async () => {
    // no action is needed
  });
});

Reference: https://sinonjs.org/releases/v6.3.5/stubs/#stubresolvesvalue

I hope this offers some assistance

Answer 2

The two scenarios are not the same.

When looking at the code snippet, it's clear that in the first case JS immediately executes the statement Promise.reject as an argument, resulting in the early warning UnhandledPromiseRejectionWarning.

const failApiClient = getFakeApi(Promise.reject(new Error()));

In contrast,

const explicitFailApiClient = {
  getObjects(): Promise<LocalObject[]> {
    return Promise.reject(new Error());
  }
};

The Promise.reject in this second scenario is evaluated only when

explicitFailApiClient.getObjects()

is invoked.

A Different Approach

To address this issue differently, another solution can be implemented using Sinon's resolves and rejects.

const getFakeApi = {getObjects: (result) => result};
const getFakeApiStub = sinon.stub(getFakeApi, 'getObjects');

describe('successApiClient', () => {
  before(() => {
    getFakeApiStub.resolves([fakeObject]); // success and resolves
  });

  it('avoids triggering a warning', async () => {
    // no action is required
  });

});

describe('failApiClient', () => {
  before(() => {
    getFakeApiStub.rejects(new Error()); // force it to fail
  });

  it('should not raise a warning', async () => {
    // no action is needed
  });
});

Reference: https://sinonjs.org/releases/v6.3.5/stubs/#stubresolvesvalue

I hope this offers some assistance

Answer 3

Answer №2

There are three possible outcomes for promises: unresolved, resolved, or rejected. The "then" method is used to handle a resolution, while the "catch" method is used to handle a rejection. It seems like you're throwing a rejection without catching it.

You can try

SomePromiseRejection().catch(err => DoSomeStuff(err))

Therefore, make sure to include a .catch block after calling explicitFailApiClient.

Answer 4

There are three possible outcomes for promises: unresolved, resolved, or rejected. The "then" method is used to handle a resolution, while the "catch" method is used to handle a rejection. It seems like you're throwing a rejection without catching it.

You can try

SomePromiseRejection().catch(err => DoSomeStuff(err))

Therefore, make sure to include a .catch block after calling explicitFailApiClient.

Sporadic UnhandledPromiseRejectionWarning surfacing while utilizing sinon

Answer №1

A Different Approach

Answer №2

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npm Error: The installation script for [email protected] has encountered a failure