CODEWITHSUNDEEP
javascriptpromise
iamthegreenfairy
iamthegreenfairy

Reputation: 1396

How do you properly chain promises in Javascript, and account for error handling?

I am having some trouble understanding Javascript promises, specifically chaining them and passing errors up the chain. In the following code: 

function myPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(function() {
      console.log('done')
      reject('resolved');
    }, 1000);
  });
}

function myOtherPromise() {
  return new Promise((resolve, reject) => {
    myPromise().then(done => {
      resolve(done);
    }).catch(e => {
      console.log('In myOtherPromise, caught err: ', e);
      reject(e)
    });
  });
}

myOtherPromise().then(done => {
  console.log('done calling myOtherPromise: ', done);
}).catch(e => {
  console.log('caught err from myOtherPromise', err);
});

The output shows:

done
In myOtherPromise, caught err:  resolved

I don't understand why the following is not printed:

'caught err from myOtherPromise'

I feel like there is something fundamental I am not quite getting. Why doesn't the rejection from myOtherPromise get passed to the final catch block?

Upvotes: 0

Views: 65

Answers (1)

Ruslan Stelmachenko
Ruslan Stelmachenko

Reputation: 5439

You catch the error into the e variable, but output the err variable (which is undefined and causes runtime error).

.catch(e => {
  console.log('caught err from myOtherPromise', err);
})

Should be:

.catch(e => {
  console.log('caught err from myOtherPromise', e);
})

Upvotes: 4

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