Promise: Ignore Catch and Return to Chain

Question

Is it possible to ignore a catch and return back to the chain?

promiseA()        // <-- fails with 'missing' reason
  .then(promiseB) // <-- these are not going to run 
  .then(promiseC)
  .catch(function(error, ignore){
     if(error.type == 'missing'){
        ignore()  // <-- ignore the catch and run promiseB and promiseC
     }
  })

Is something like this possible?

`.then` has 2 arguments.. `fulfill` and `reject`.. You can deal with `reject` — Rayon, Apr 13 '16 at 11:28
@RayonDabre Thanks for your answer but I'm not really sure what do you mean. Please can you show an example? — Adam Halasz, Apr 13 '16 at 11:31
It takes two functions as arguments: then(onFulFill, onReject) — Matt, Apr 13 '16 at 11:35

Madara's Ghost · Accepted Answer · 2018-05-09T18:03:52.100

Here's the synchronous analogy:

try {
  action1(); // throws
  action2(); // skipped
  action3(); // skipped
} catch (e) {
  // can't resume
}

vs

try {
  action1(); // throws
} catch (e) {
  handleError(e);
}
action2(); // executes normally
action3();

Here's the promise version:

asyncActionA()        // <-- fails with 'missing' reason
.catch(error => {
   if(error.type == 'missing'){
      return; // Makes sure the promise is resolved, so the chain continues
   }
   throw error; // Otherwise, rethrow to keep the Promise rejected
})
.asyncActionB(promiseB) // <-- runs
.asyncActionC(promiseC)
.catch(err => {
  // Handle errors which are not of type 'missing'.
});

score 1 · Answer 2 · answered Apr 13 '16 at 11:34

1

If you need just ignore all error in promiseA, you can just do it like that:

promiseA()  
  .catch(function(error){
      //just do nothing, returns promise resolved with undefined
  })  
  .then(promiseB)
  .then(promiseC)

If you need to run promiseB only when error.type == 'missing', you can do that:

promiseA()       
  .catch(function(error, ignore){
     if(error.type == 'missing'){
        return promiseB.then(promiseC)  
     }
  })

answered Apr 13 '16 at 11:34

Nikolai Mavrenkov

1,851
18
21

2

The main point it that `catch` function also returns promise, so you can use it to implement fallback logic and chain result of it – Nikolai Mavrenkov Apr 13 '16 at 11:36
1

No nested `.then()` calls please. – Madara's Ghost Apr 13 '16 at 11:37
2

@Madara'sGhost why not? – Tim Visser Feb 06 '20 at 10:00
Nested `.then()` are ok in certain cases, e.g. if you need to make detour through side chain on some condition. Of course, one chain is more readable and cleaner, but "No nested `then`" is just wrong. – Krzysiek Jul 02 '21 at 07:44

Promise: Ignore Catch and Return to Chain

2 Answers2

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