NodeJS Promise（Q） - 如果promise失败，如何获取值？

Question

I´m starting to use promises in my NodeJS projects and I run into a problem. After I read the Promises/A+ spec and googling a lot I didn´t find a nice solution for the case that I need to access a value which gets produced in a promise chain. In my example I want to check when an error occurs if the image was created and if so, I want to delete it.

Code:

var Q = require('Q');
var fs = require('fs');

// This produces the imgPath
function makeImage() {
    var deferred = Q.defer();
    deferred.resolve("path/to/image");
    return deferred.promise;
}

function watermark(imgPath) {
    var deferred = Q.defer();
    deferred.resolve(imgPath);
    return deferred.promise;
}

// This function fails
function doSomeCoolThings(imgPath) {
    var deferred = Q.defer();
    deferred.reject(new Error("System is exploded"));
    return deferred.promise;
}

var fileExists = Q.denodeify(fs.readFile);
var deleteFile = Q.denodeify(fs.unlink);

// How do I get the imgPath here?
function deleteImageIfPresent(err) {
    return Q.fcall(function () {
        return imgPath !== undefined;
    })
        .then(fileExists)
        .then(deleteFile);
}

var iKnowThatSolution;

makeImage()
    // Thats not what I want
    //.then(function(imgPath) {
    //    iKnowThatSolution = imgPath;
    //})
    .then(watermark)
    .then(doSomeCoolThings)
    .fail(deleteImageIfPresent);

Answer 1

I recommend this approach:

return makeImage().then(function (path) {
    return doSomeCoolThings(path)
    .finally(function () {
        return removeImage(path);
    });
});

Assuming you trust makeImage, removeImage, and doSomeCoolThings to return Q promises and never throw. Otherwise, there's always Q.fcall which eliminates these concerns.

If you wish to retain the image in the success case, and only delete it if there's a failure, rethrowing the error:

return Q.fcall(makeImage).then(function (path) {
    return Q.fcall(doSomeCoolThings, path)
    .catch(function (error) {
        return Q.fcall(removeImage, path)
        .thenReject(error);
    });
});

Also, instead of:

var deferred = Q.defer();
deferred.resolve(x);
return deferred.promise;

You can:

return Q.resolve(x);

Answer 2

Instead of:

deferred.reject(new Error("System is exploded"));

You could return an object containing both the error and additional data that you need in the error handler, like so:

deferred.reject({
  error: new Error("System is exploded"),
  data: {
    imgPath: imgPath,
    ...
  }
});

Then you can access imgPath with err.data.imgPath inside deleteImageIfPresent function.

You will probably want to handle errors created in 3rd party libraries. You can catch them in one error handler and re-throw the error with an explicit reject wrapping in an object and adding the data you need.

---

Also as a side note, there is no .fail in Promises/A+. Use:

promise.then(onFulfilled, onRejected);

Not technically an error but since you mentioned it.