如何将以下JavaScript Promise转换为常规的异步代码？

Question

Hi I'm working with the following code snippet:

imaps.connect(config).then(function (connection) {

connection.openBox('INBOX').then(function () {

    // Fetch emails from the last 24h
    var delay = 24 * 3600 * 1000;
    var yesterday = new Date();
    yesterday.setTime(Date.now() - delay);
    yesterday = yesterday.toISOString();
    var searchCriteria = ['UNSEEN', ['SINCE', yesterday]];
    var fetchOptions = { bodies: ['HEADER.FIELDS (FROM TO SUBJECT DATE)'], struct: true };

    // retrieve only the headers of the messages
    return connection.search(searchCriteria, fetchOptions);
}).then(function (messages) {

    var attachments = [];

    messages.forEach(function (message) {
        var parts = imaps.getParts(message.attributes.struct);
        attachments = attachments.concat(parts.filter(function (part) {
            return part.disposition && part.disposition.type.toUpperCase() === 'ATTACHMENT';
        }).map(function (part) {
            // retrieve the attachments only of the messages with attachments
            return connection.getPartData(message, part)
                .then(function (partData) {
                    return {
                        filename: part.disposition.params.filename,
                        data: partData
                    };
                });
        }));
    });

    return Promise.all(attachments);
}).then(function (attachments) {
    console.log(attachments);
    // =>
    //    [ { filename: 'cats.jpg', data: Buffer() },
    //      { filename: 'pay-stub.pdf', data: Buffer() } ]
});

I'm trying to remove Promises and turn the code into code using callbacks.

At the moment, I'm looping through all the attachments and when I print an individual attachment to the console I get:

Promise { <pending> }

How can I convert the code to regular callback code such that I can access the 'filename' and 'data' attributes of the attachment?

Answer 1

Actually, I don't understand why you need to go back from a promise to a callback but you can use the following code. It just a promise converter:

const promiseToCallback = function (promise) {
    if (!isFn(promise.then)) {
        throw new TypeError('Expected a promise');
    }

    return function (cb) {
        promise.then(function (data) {
            setImmediate(cb, null, data);
        }, function (err) {
            setImmediate(cb, err);
        });
    };
};

To use it

promiseToCallback(promise)(function(err, data) {
    ...
});