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

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);    })}

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();}

I believe you only have to reject the promise to break out of the promise chain.

https://github.com/kriskowal/q/wiki/API-Reference#qrejectreason

also it seems like .end() has been changed to .done()

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!');            // by calling Q.reject, your second .then is skipped,            // only the .fail is executed.            // result1 will be passed to your callback in the .fail call            return Q.reject(result1);        }        return Q.ncall(task.step2, task);    })    .then(function(result2){        console.log('doing step 3...');        return Q.ncall(task.step3, task);    })    .fail(callback).done();}