how to break promise chain

Question

I a promise in such fashion,

function getMode(){
    var deferred = Promise.defer();

    checkIf('A')
    .then(function(bool){
        if(bool){
            deferred.resolve('A');
        }else{
            return checkIf('B');
        }
    }).then(function(bool){
        if(bool){
            deferred.resolve('B');
        }else{
            return checkIf('C');
        }
    }).then(function(bool){
        if(bool){
            deferred.resolve('C');
        }else{
            deferred.reject();
        }
    });

    return deferred.promise;
}

checkIf returns a promise, and yes checkIf cannot be modified.

How do I break out of the chain at the first match? (any way other than explicitly throwing error?)

Answer 1

Any way other than explicitly throwing error?

You may need to throw something, but it does not have to be an error.

Most promise implementations have method catch accepting the first argument as error type (but not all, and not ES6 promise), it would be helpful under this situation:

function BreakSignal() { }

getPromise()
    .then(function () {
        throw new BreakSignal();
    })
    .then(function () {
        // Something to skip.
    })
    .catch(BreakSignal, function () { })
    .then(function () {
        // Continue with other works.
    });

I add the ability to break in the recent implementation of my own promise library. And if you were using ThenFail (as you would probably not), you can write something like this:

getPromise()
    .then(function () {
        Promise.break;
    })
    .then(function () {
        // Something to skip.
    })
    .enclose()
    .then(function () {
        // Continue with other works.
    });