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

node.js, promise, q

Solution

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();
}

Problem

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!

Original source