Getting a UnhandledPromiseRejectionWarning when testing using mocha chai

Question

So  I m testing a component that relies on an event-emitter  To do so I came up with a solution using Promises with Mocha Chai  it  should transition with the correct event    done    gt      const cFSM   new CharacterFSM     emitter  transitions     let timeout   null    let resolved   false    new Promise  resolve  reject    gt        emitter once  action   resolve       emitter emit  done            timeout   setTimeout      gt          if   resolved            reject  Timedout                   clearTimeout timeout          100        then  state    gt        resolved   true      assert state action      DONE    should change state        done         catch  error    gt        assert isNotOk error  Promise error        done               On the console I m getting an  UnhandledPromiseRejectionWarning  even though the reject function is getting called since it instantly shows the message  AssertionError  Promise error    node 25754  UnhandledPromiseRejectionWarning  Unhandled promise rejection  rejection id   2   AssertionError  Promise error  expected   Object  message  showDiff         to be falsy  should transition with the correct event   And then  after 2 sec I get  Error  timeout of 2000ms exceeded  Ensure the done   callback is being called in this test   Which is even weirder since the catch callback was executed I think that for some reason the assert failure prevented the rest of the execution  Now the funny thing  if I comment out the assert isNotOk error     the test runs fine without any warning in the console  It stills  fails  in the sense that it executes the catch  But still  I can t understand these errors with promise  Can someone enlighten me

User · Answer

I got this error when stubbing with sinon.

The fix is to use npm package sinon-as-promised when resolving or rejecting promises with stubs.

Instead of ...

sinon.stub(Database, 'connect').returns(Promise.reject( Error('oops') ))

Use ...

require('sinon-as-promised');
sinon.stub(Database, 'connect').rejects(Error('oops'));

There is also a resolves method (note the s on the end).

See http://clarkdave.net/2016/09/node-v6-6-and-asynchronously-handled-promise-rejections

User · Answer

Here s my take experience with E7 async await  In case you have an async helperFunction   called from your test     one explicilty with the ES7 async keyword  I mean    make sure  you call that as await helperFunction whateverParams    well  yeah  naturally  once you know     And for that to work  to avoid    await is a reserved word      your test-function must have an outer async marker  it  my test   async      gt

User · Answer

For those who are looking for the error warning UnhandledPromiseRejectionWarning outside of a testing environment  It could be probably because nobody in the code is taking care of the eventual error in a promise   For instance  this code will show the warning reported in this question   new Promise  resolve  reject    gt      return reject  Error reason            node XXXX  UnhandledPromiseRejectionWarning  Unhandled promise rejection  rejection id  1   Error  Error reason   and adding the  catch   or handling the error should solve the warning error  new Promise  resolve  reject    gt      return reject  Error reason        catch      gt       do whatever you want here          Or using the second parameter in the then function  new Promise  resolve  reject    gt      return reject  Error reason        then null       gt       do whatever you want here

User · Answer

I had a similar experience with Chai-Webdriver for Selenium   I added await to the assertion and it fixed the issue   Example using Cucumberjs   Then  I see heading with the text of Tasks   async function         await chai expect  h1   dom to contain text  Tasks

User · Answer

The assertion libraries in Mocha work by throwing an error if the assertion was not correct  Throwing an error results in a rejected promise  even when thrown in the executor function provided to the catch method    catch  error    gt      assert isNotOk error  Promise error      done          In the above code the error objected evaluates to true so the assertion library throws an error    which is never caught  As a result of the error the done method is never called  Mocha s done callback accepts these errors  so you can simply end all promise chains in Mocha with  then done done   This ensures that the done method is always called and the error would be reported the same way as when Mocha catches the assertion s error in synchronous code   it  should transition with the correct event    done    gt      const cFSM   new CharacterFSM     emitter  transitions     let timeout   null    let resolved   false    new Promise  resolve  reject    gt        emitter once  action   resolve       emitter emit  done            timeout   setTimeout      gt          if   resolved            reject  Timedout                   clearTimeout timeout          100        then   state    gt        resolved   true      assert state action      DONE    should change state          then done done         I give credit to this article for the idea of using  then done done  when testing promises in Mocha

User · Answer

The issue is caused by this    catch  error    gt      assert isNotOk error  Promise error      done          If the assertion fails  it will throw an error  This error will cause done   never to get called  because the code errored out before it  That s what causes the timeout   The  Unhandled promise rejection  is also caused by the failed assertion  because if an error is thrown in a catch   handler  and there isn t a subsequent catch   handler  the error will get swallowed  as explained in this article   The UnhandledPromiseRejectionWarning warning is alerting you to this fact   In general  if you want to test promise-based code in Mocha  you should rely on the fact that Mocha itself can handle promises already  You shouldn t use done    but instead  return a promise from your test  Mocha will then catch any errors itself   Like this   it  should transition with the correct event        gt            return new Promise  resolve  reject    gt                 then  state    gt        assert state action      DONE    should change state            catch  error    gt        assert isNotOk error  Promise error

User · Answer

I faced this issue       node 1131004  UnhandledPromiseRejectionWarning  Unhandled promise rejection  re   jection id  1   TypeError  res json is not a function    node 1131004  DeprecationWarning  Unhandled promise rejections are deprecated    In the future  promise rejections that are not handled will terminate the Node j   s process with a non-zero exit code    It was my mistake  I  was replacing res object in then function res   so changed res to result and now it is working   Wrong  module exports update   function req  res           return Services User update req body                   then function res    issue was here  res overwrite                     return res json res                      function error                       return res json  error error message                       catch function                         console log  Promise Rejected                        Correction  module exports update   function req  res           return Services User update req body                   then function result    res replaced with result                     return res json result                      function error                       return res json  error error message                       catch function                         console log  Promise Rejected                        Service code   function update data      var id   new require  mongodb   ObjectID data  id           userData                         name data name                      email data email                      phone  data phone                     return collection findAndModify              id id      query               id   asc         sort order             set  userData      replacement              new   true               then function doc                    if  doc                      throw new Error  Record not updated                     return doc value                          module exports             update update

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