How to return from a Promise's catch/then block?

Question

There are many tutorials on how to use \"then\" and \"catch\" while programming with JavaScript Promise. However, all these tutorials seem to miss an important point: return

Answer 1

There is no built-in functionality to skip the entirety of the remaining chain as you're requesting. However, you could imitate this behavior by throwing a certain error through each catch:

doSomething()
  .then(func1).catch(handleError)
  .then(func2).catch(handleError)
  .then(func3).catch(handleError);

function handleError(reason) {
  if (reason instanceof criticalError) {
    throw reason;
  }

  console.info(reason);
}

If any of the catch blocks caught a criticalError they would skip straight to the end and throw the error. Any other error would be console logged and before continuing to the next .then block.

Answer 2

This can't be achieved with features of the language. However, pattern-based solutions are available.

Here are two solutions.

Rethrow previous error

This pattern is basically sound ...

Promise.resolve()
.then(Function1).catch(errorHandler1)
.then(Function2).catch(errorHandler2)
.then(Function3).catch(errorHandler3)
.then(Function4).catch(errorHandler4)
.catch(finalErrorHandler);

Promise.resolve() is not strictly necessary but allows all the .then().catch() lines to be of the same pattern, and the whole expression is easier on the eye.

... but :

• if an errorHandler returns a result, then the chain will progress to the next line's success handler.
• if an errorHandler throws, then the chain will progress to the next line's error handler.

The desired jump out of the chain won't happen unless the error handlers are written such that they can distinguish between a previously thrown error and a freshly thrown error. For example :

function errorHandler1(error) {
    if (error instanceof MyCustomError) { // <<<<<<< test for previously thrown error 
        throw error;
    } else {
        // do errorHandler1 stuff then
        // return a result or 
        // throw new MyCustomError() or 
        // throw new Error(), new RangeError() etc. or some other type of custom error.
    }
}

Now :

• if an errorHandler returns a result, then the chain will progress to the next FunctionN.
• if an errorHandler throws a MyCustomError, then it will be repeatedly rethrown down the chain and caught by the first error handler that does not conform to the if(error instanceof MyCustomError) protocol (eg a final .catch()).
• if an errorHandler throws any other type of error, then the chain will progress to the next catch.

This pattern would be useful if you need the flexibility to skip to end of chain or not, depending on the type of error thrown. Rare circumstances I expect.

DEMO

Insulated Catches

Another solution is to introduce a mechanism to keep each .catch(errorHandlerN) "insulated" such that it will catch only errors arising from its corresponding FunctionN, not from any preceding errors.

This can be achieved by having in the main chain only success handlers, each comprising an anonymous function containing a subchain.

Promise.resolve()
.then(function() { return Function1().catch(errorHandler1); })
.then(function() { return Function2().catch(errorHandler2); })
.then(function() { return Function3().catch(errorHandler3); })
.then(function() { return Function4().catch(errorHandler4); })
.catch(finalErrorHandler);

Here Promise.resolve() plays an important role. Without it, Function1().catch(errorHandler1) would be in the main chain the catch() would not be insulated from the main chain.

Now,

• if an errorHandler returns a result, then the chain will progress to the next line.
• if an errorHandler throws anything it likes, then the chain will progress directly to the finalErrorHandler.

Use this pattern if you want always to skip to the end of chain regardless of the type of error thrown. A custom error constructor is not required and the error handlers do not need to be written in a special way.

DEMO

Usage cases

Which pattern to choose will determined by the considerations already given but also possibly by the nature of your project team.

• One-person team - you write everything and understand the issues - if you are free to choose, then run with your personal preference.
• Multi-person team - one person writes the master chain and various others write the functions and their error handlers - if you can, opt for Insulated Catches - with everything under control of the master chain, you don't need to enforce the discipline of writing the error handlers in that certain way.

Answer 3

First off, I see a common mistake in this section of code that could be completely confusing you. This is your sample code block:

Promise.resolve(someFunction()).then(function() {
  console.log('someFunction should throw error');
  return -2;
}).catch(function(err) {
   if (err instanceof MyCustomError) {
     return -1;
   }
}).then(someOtherFunction());

You need pass function references to a .then() handler, not actually call the function and pass their return result. So, this above code should probably be this:

Promise.resolve(someFunction()).then(function() {
  console.log('someFunction should throw error');
  return -2;
}).catch(function(err) {
   if (err instanceof MyCustomError) {
     // returning a normal value here will take care of the rejection
     // and continue subsequent processing
     return -1;
   }
}).then(someOtherFunction);    // just pass function reference here

Note that I've removed () after the functions in the .then() handler so you are just passing the function reference, not immediately calling the function. This will allow the promise infrastructure to decide whether to call the promise in the future or not. If you were making this mistake, it will totally throw you off for how the promises are working because things will get called regardless.

---

Three simple rules about catching rejections.

1. If nobody catches the rejection, it stops the promise chain immediately and the original rejection becomes the final state of the promise. No subsequent handlers are invoked.
2. If the promise rejection is caught and either nothing is returned or any normal value is returned from the reject handler, then the reject is considered handled and the promise chain continues and subsequent handlers are invoked. Whatever you return from the reject handler becomes the current value of the promise and it as if the reject never happened (except this level of resolve handler was not called - the reject handler was called instead).
3. If the promise reject is caught and you either throw an error from the reject handler or you return a rejected promise, then all resolve handlers are skipped until the next reject handler in the chain. If there are no reject handlers, then the promise chain is stopped and the newly minted error becomes the final state of the promise.

---

You can see a couple examples in this jsFiddle where it shows three situations:

1. Returning a regular value from a reject handler, causes the next .then() resolve handler to be called (e.g. normal processing continues),

2. Throwing in a reject handler causes normal resolve processing to stop and all resolve handlers are skipped until you get to a reject handler or the end of the chain. This is effective way to stop the chain if an unexpected error is found in a resolve handler (which I think is your question).

3. Not having a reject handler present causes normal resolve processing to stop and all resolve handlers are skipped until you get to a reject handler or the end of the chain.