节点-async.map如何工作？

Question

After reading about async , I assumed the code below would output to the console the total of all values returned from the http/API call; but it seems to fire immediately after the first http call returns, and only shows a 'total' value equal to the first value returned from the API.

Where is my misunderstanding about how async.map works?

var http = require('https');
const
async = require('async');

var MongoClient = require('mongodb').MongoClient;
var dbUrl = "mongodb://localhost:27017/";

var total = 0;

var tokens = [ {
    name : "tron"
}, {
    name : 'cardano'
}, {
    name : 'nucleus-vision'
}, {
    name : 'ripple'
}, {
    name : 'litecoin'
}, {
    name : 'havven'
}];

function run() {

    doStuff();
    setInterval(doStuff, 1 * 60 * 1000);
};

function doStuff() {

    total = 0;

    async.map(tokens, httpGet, function (value){
          console.log('async done ', total);
        });

}

function httpGet(token, callback) {

    var url = 'https://api.coinmarketcap.com/v1/ticker/' + token.name;
    http.get( url,
        function(res) {
            var body = '';

            res.on('data', function(chunk) {
                body += chunk;
            });

            res.on('end', function() {
                var jsonObj = JSON.parse(body);
                var price = parseFloat(jsonObj[0].price);

                total += price;

                MongoClient.connect(dbUrl, function(err, db) {
                    if (err)
                        throw err;
                    var dbo = db.db("crypto");
                    dbo.collection("tick").insertOne(jsonObj[0],
                            function(err, res) {
                                if (err)
                                    throw err;
                                db.close();

                            });
                });


                callback(price);
            });



        }).on('error', function(e) {
        console.log("Got an error: ", e);
    });

};


run();

Answer 1

callback that is passed to an iteratee ( httpGet ) is used incorrectly. The first argument ( price ) is considered an error. From the docs :

If iteratee passes an error to its callback, the main callback (for the map function) is immediately called with the error.

So

callback(price);

should rather be

callback(null, price);

So async does not halt after the first iteration.

Answer 2

I believe there are two separate problems here:

1. As you may know, we cannot use return statements in asynchronous code like we would in synchronous code, which is why we use callbacks instead. Node-style callbacks are on the form function (err, result) {} , where the first parameter is the error (if any) and the second the result of the function (the return value). According to the docs , Async.map(coll, iteratee, callback) will stop the execution if the iteratee passes an error to its callback.

   As your iteratee -function is calling its callback as such: callback(price) , you're effectively stopping execution, as price is passed as the error parameter. What you want to do to "return" the price variable, is to call the callback as so: callback(null, price)

2. Typically, map -functions are used for

   appl[ying] a given function to each element of a list, returning a list of results in the same order.

   The map function of the async library does the same, IE: iterates through an array and returns an array of the resulting items, just like the normal map (below) method does.

   [1, 2, 3].map(function (nbr) { return nbr*2 }) // returns [2, 4, 6]

   The result parameter of your callback (IE, the third parameter to async.map) will be called with an array of prices, and not the summed value of the prices.

   async.map(tokens, httpGet, function (error, total) { console.log(error); // prints undefined (unless there was an error) console.log(total); // prints an array of prices });

For summing the values, I would recommend the reduce function, or simply sum the values returned as a result.