How to properly abort a node.js promise chain using Q?

Question

I'm using the Q module for Node.js in attempts to avoid the "pyramid of doom" in scenarios where I have many steps. For example:

function doTask(task, callback) {     Q.ncall(task.step1, task)     .then(function(result1){         return Q.ncall(task.step2, task);     })     .then(function(result2){         return Q.ncall(task.step3, task);     })     .fail(callback).end(); } 

Essentially this seems to work; if an error is thrown by any of the task steps, it is passed to the callback (though I would be welcome to improvements, as I am new to node.js promises). However, I have a problem when I need to abort the task-chain early. For example, if result1 is successfully returned I might want to call the callback early and abort the rest, but my attempts to do so are failing...

function doTask(task, callback) {     Q.ncall(task.step1, task)     .then(function(result1){         if(result1)         {// the rest of the task chain is unnecessary              console.log('aborting!');             callback(null, result1);             return null;         }         return Q.ncall(task.step2, task);     })     .then(function(result2){         console.log('doing step 3...');         return Q.ncall(task.step3, task);     })     .fail(callback).end(); } 

In this example, I see both "aborting!" and "doing step 3..." printed.

I'm sure I'm merely misunderstanding some basic principles here, so would appreciate any help. Thanks!

Answer 1

This is a case where you will need to branch, which does mean either nesting or creating a subroutine.

function doTask(task, callback) {     return Q.ncall(task.step1, task)     .then(function(result1) {         if (result1) return result1;         return Q.ncall(task.step2, task)         .then(function(result2) {             return Q.ncall(task.step3, task);         })     })     .nodeify(callback) } 

Or

function doTask(task, callback) {     return Q.ncall(task.step1, task)     .then(function(result1) {         if (result1) {             return result1;         } else {             return continueTasks(task);         }     })     .nodeify(callback) }  function continueTasks(task) {     return Q.ncall(task.step2, task)     .then(function(result2) {         return Q.ncall(task.step3, task);     }) } 

Answer 2

Any errors that are thrown within the promise chain will cause the entire stack to be aborted early and control is given to the error-back path. (in this case, the fail() handler) When you detect a certain state which causes you to want to abort the promise chain, then just throw a very specific error, which you trap in the error-back and ignore (if you so choose)

function doTask(task, callback) {     Q.ncall(task.step1, task)     .then(function(result1){         if(result1 == 'some failure state I want to cause abortion')         {// the rest of the task chain is unnecessary              console.log('aborting!');             throw new Error('abort promise chain');             return null;         }         return Q.ncall(task.step2, task);     })     .then(function(result2){         console.log('doing step 3...');         return Q.ncall(task.step3, task);     })     .fail(function(err) {         if (err.message === 'abort promise chain') {             // just swallow error because chain was intentionally aborted         }         else {             // else let the error bubble up because it's coming from somewhere else             throw err;         }      })     .end(); }