node.js承诺不强制执行功能

Question

I have three functions that I want to use promises to force them to execute in order.

function 1 sends a http request, fetches JSON data and saved it to a file function 2 loops through that file and updates the database according the difference values/values missing function 3 will loop through the newly updated database and create a 2nd json file.

Currently function 1 works perfectly on its own with a setInterval of 30 minutes.

I want to start function 2 when function 1 has finished. then function 3 after function 2 has finished.

Using promises I am trying to attach function 2 to a simple finished log to understand how to use promises but not getting much success. The items from the for loop log but my Finished/err log before my for loop which shouldn't be happening. Any suggestions?

function readJson() {
    return new Promise(function() {
        fs.readFile(__dirname + "/" + "bitSkin.json", 'utf8', function read(err, data) {
            if (err) { throw err; }
            var bitCon = JSON.parse(data);  

            for(var i=0; i<7; i++) { //bitCon.prices.length; i++) {
                var price = bitCon.prices[i].price
                var itemName = bitCon.prices[i].market_hash_name;
                (function() {
                    var iNameCopy = itemName;
                    var priceCopy = price;
                    logger.info(iNameCopy);
                }());
            }
        });
    });
};

function fin() {
    logger.info("Finished");
}

readJson().then(fin(), console.log("err"));

Answer 1

Promises have no magical powers. They don't magically know when async code inside them is done. If you create a promise, you yourself have to resolve() or reject() it when the async code has an error or completes.

Then, in addition, you have to pass a function reference to a .then() handler, not the result of executing a function. .then(fin()) will call fin() immediately and pass it's return value to .then() which is not what you want. You want something like .then(fin) .

Here's how you can resolve and reject the promise you created:

function readJson() {
    return new Promise(function(resolve, reject) {
        fs.readFile(__dirname + "/" + "bitSkin.json", 'utf8', function read(err, data) {
            if (err) { return reject(err); }
            var bitCon = JSON.parse(data);  

            for(var i=0; i<7; i++) { //bitCon.prices.length; i++) {
                var price = bitCon.prices[i].price
                var itemName = bitCon.prices[i].market_hash_name;
                (function() {
                    var iNameCopy = itemName;
                    var priceCopy = price;
                    logger.info(iNameCopy);
                }());
            }
            resolve(bitCon);
        });
    });
};

And, you could use that like this:

function fin() {
    logger.info("Finished");
}
readJson().then(fin, function(err) {
    console.log("err", err)
});

Summary of changes:

1. Added resolve, reject arguments to Promise callback so we can use them
2. Called reject(err) when there's an error
3. Called resolve() when the async code is done.
4. Passed a function reference for both .then() handlers.

---

FYI, when creating a promise wrapper around an async function, it is generally better to wrapper just the function itself. This makes the wrapper 100% reusable and puts more of your code in the promise architecture which generally streamlines things and makes error handling easier. You could fix things up that way like this:

fs.readFilePromise = function(file, options) {
    return new Promise(function(resolve, reject) {
        fs.readFile(file, options, function(err, data) {
            if (err) return reject(err);
            resolve(data);
        });
    });
});

function readJson() {
    return fs.readFilePromise(__dirname + "/" + "bitSkin.json", 'utf8').then(function(data) {
        var bitCon = JSON.parse(data);
        bitCon.prices.forEach(function(item) {
            logger.info(item.market_hash_name);
        });
        return bitCon;
    });
}