Node js for-loop wait for asynchronous function before next iteration?

问题

I am making a function that refreshes data every so often and I am having issues with the request chain that I have. The problem is that I have a for-loop running the asynchronous requests and the for-loop will finish before the requests are done.

setInterval(function(){ // this updates the total hours of all members every 10 seconds
    request({ // this gets all of the loyalty program members
        url: "",//omitted
        method: "GET"
    },
        function(listError, listResponse, listBody) {
            if(listError == null && listResponse.statusCode == 200) {
                var varBody = {};
                var listObj = JSON.parse(listBody);
                for(var i = 0; i < listObj.result.length; i++) { // parses through all of the members to update their hours

                    console.log(i);//****PRINT STATEMENT

                    varBody.index = i;
                    varBody.memberID = listObj.result[i].program_member.id;
                    request({ //we do this request to get the steam ID of the program member
                            url: "",//omitted
                            method: "GET"
                        },
                        function(fanError, fanResponse, fanBody) {

                            var fan = JSON.parse(fanBody);
                            if(fanError == null && fanResponse.statusCode == 200 && fan.result.profiles.length != 0) { // make sure that the profile isn't empty
                                request({
                                        url:"",//omitted
                                        method: "GET"
                                    },
                                    function(hourError, hourResponse, hourBody) {
                                        if (hourError == null && hourResponse.statusCode == 200) {
                                            var gameList = JSON.parse(hourBody);
                                            var minutes = 0;
                                            for (var j = 0; j < gameList.response.games.length; j++) { // for loop to calculate the minutes each user has on steam
                                                minutes += gameList.response.games[j].playtime_forever;
                                            }
                                            var deltaHours = 1;
                                            if(deltaHours != 0) {
                                                var transaction = { // updated member object to be inserted
                                                    pointsearned: deltaHours,
                                                    pointsused: 0,
                                                    loyaltyprogram_id: loyaltyID,
                                                    programmember_id: memberID
                                                };
                                                request({ // POST request to update the member
                                                        url: "",//omitted
                                                        method: "POST",
                                                        body: JSON.stringify(transaction),
                                                        headers: {
                                                            "Content-Type": "application/json"
                                                        }
                                                    },
                                                    function(updateError, updateRes, updateBody) {
                                                        if(updateError == null && updateRes.statusCode == 200) {
                                                            console.log("Success");//****PRINT STATEMENT
                                                        }
                                                    }
                                                );
                                            }
                                        }
                                    }
                                );
                            }
                        }
                    );
                }
            }
            console.log("Users Updated"); //****PRINT STATEMENT
        }
    );
}, 10000);

If I were to run this code, it would print:

0
1
2
3
Success
Success
Success
Success

I know what the issue is. It's the fact that the for-loop doesn't wait for the requests to finish. What I don't know is a work-around for this. Does anyone have any ideas?

回答1:

You want the async library.

For instance,

for(var i = 0; i < listObj.result.length; i++) {
    varBody.index = i;
    varBody.memberID = listObj.result[i].program_member.id;
    request(
        ...
    , function () {
        // Do more Stuff
    });
}

Can be written like this instead:

async.forEachOf(listObj.result, function (result, i, callback) {
    varBody.index = i;
    varBody.memberID = result.program_member.id;
    request(
        ...
    , function () {
        // Do more Stuff
        // The next iteration WON'T START until callback is called
        callback();
    });
}, function () {
    // We're done looping in this function!
});

There are lots of handy utility functions like this in async that makes working with callbacks much much easier.

回答2:

For completeness, the way to do async things sequentially "by hand" is to use recursion:

function dothings(things, ondone){
    function go(i){
        if (i >= things.length) {
            ondone();
        } else {
            dothing(things[i], function(result){
                return go(i+1);
            });
        }
    }
    go(0);
}

来源：https://stackoverflow.com/questions/38773912/node-js-for-loop-wait-for-asynchronous-function-before-next-iteration