如何打破承诺链？

Question

I'm aware that there are similar questions, but I haven't seen any that address this chaining pattern.

I have the following:

    var runTests = function (chain, resolutionTest) {
        return chain.then(function (result) {
            if (result)
                return result; // Early return if the previous tests were successful.  This is where I want to prevent other attempts.
            const attempt = tryOpenStream(resolutionTest).then(streamToDom);
            return attempt;
        });
    }

    // from someplace else
    numTests = resolutionTests.length;
    return resolutionTests.reduce(runTests, Promise.resolve()); // start reduce with an empty promise 

The problem that I'm encountering is that I'm calling tryOpenStream many times even after I've captured a result .

Options I'm considering:

• Raise some global flag that just prevents further execution from within the chain. Yuck, because the chain still proceeds, it's just emptied.
• throw new Error(result) instead of return result . This would break the chain (I think...) but it's misusing Error and would be easily misunderstood by another developer.

How can I break this chain at return result; ?

UPDATE 1

I'm trying the following:

   var makeTest = function (runMoreTests, resolutionTest) {
        return function runTest() {
            return tryOpenStream(resolutionTest).then(streamToDom).then(function (result) {
                if (result)
                    return result;
                else
                    return runMoreTests();
            });
        };
    }

    return resolutionTestBuilder.buildTests().then(function (resolutionTests) {
        numTests = resolutionTests.length;
        return resolutionTests.reduceRight(makeTest, function () { Promise.reject("No resolutions succeeded.") })();
    });

However no calls to runTest are invoked. This is a bit of new syntax for me so I'll research some and update with any findings.

UPDATE 2

I was missing the () to invoke the reduceRight . Though now I'm seeing that reject called even with success... though when I step through, that rejection isn't invoked. It's as if by the time I get a result back, all links in the chain have been invoked.

Answer 1

Try using this:

function runTests(resolutionTest) {
    return tryOpenStream(resolutionTest).then(streamToDom)
}

// from someplace else
function loop(tests, i) {
    if (i === tests.length) return undefined
    return runTests(tests[i]).then(function (result) {
        if (result) return result
        return loop(tests, i + 1)
    })
}
return loop(resolutionTests, 0)

Although I do wonder why you can't use an exception to denote your tryOpenStream failed. That would actually simplify your code some.

Answer 2

Both flags and exceptions can be used, but as you noticed they're not the proper tool.

Instead, use recursion, like in @IsiahMeadows' answer, or a right fold:

var makeTest = function (runMoreTests, resolutionTest) {
    return function runTest(result) {
        if (result)
            return result;
        return tryOpenStream(resolutionTest).then(streamToDom).then(runMoreTests);
    };
}

return Promise.resolve(resolutionTests.reduceRight(makeTest, x => x)(undefined));

or better written as

var makeTest = function (runMoreTests, resolutionTest) {
    return function runTest() {
        return tryOpenStream(resolutionTest).then(streamToDom).then(result => {
            if (result)
                return result;
            else
                return runMoreTests();
        });
    };
}

return resolutionTests.reduceRight(makeTest, () => Promise.reject("nothing succeeded"))();