Asynchronous calls and recursion with Node.js

Question

I want to make a callback when the recursive function is complete, which can continue for an indefinite time. I am struggling with asynchronous problems and was hoping to get some help here. The code, using the module request, is as follows:

var start = function(callback) {
  request.get({
    url: 'aaa.com'
  }, function (error, response, body) {
    var startingPlace = JSON.parse(body).id;
    recurse(startingPlace, callback);
  });
};

var recurse = function(startingPlace, callback) {
    request.get({
        url: 'bbb'
    }, function(error, response, body) {
        // store body somewhere outside these funtions
        // make second request
        request.get({
            url: 'ccc'
        }, function(error, response, body) {
            var anArray = JSON.parse(body).stuff;
            if (anArray) {
                anArray.forEach(function(thing) {
                    request.get({
                        url: 'ddd'
                    }, function(error, response, body) {
                        var nextPlace = JSON.parse(body).place;
                        recurse(nextPlace);
                    });
                })
            }
        });
    });
    callback();
};

start(function() {
    // calls final function to print out results from storage that gets updated each     recursive call
    finalFunction();
});

It seems that when my code goes through the loop forin nested queries, it continues from the query and completes the initial function call while the recursive calls still continue. I want him to not complete the highest level iteration until all the nested recursive calls have completed (that I have no way of knowing how many there are).

Any help is much appreciated!

Answer 1

. , , recurse(point, otherFunc); - .

( ) ( , , ):

function recurse(startingPlace, otherFunc, callback_one) {
    // code you may have ...
    if (your_terminating_criterion === true) {
         return callback_one(val); // where val is potentially some value you want to return (or a json object with results)
    }
    // more code you may have
}

, , ( ):

recurse(startingPlace, otherFunc, function (results) {
    // results is now a variable with the data returned at the end of recursion
    console.log ("Recursion finished with results " + results);
    callback();   // the callback that you wanted to call right from the beginning
});

. , node. node . , . :

var start = function(callback) {
  request.get({
    url: 'aaa.com'
  }, function (error, response, body) {
    var startingPlace = JSON.parse(body).id;
    recurse(startingPlace, otherFunc, function (results) {
        console.log ("Recursion finished with results " + results);
        callback();
    });
  });
};

, . .

, node.js, , , . . results

return callback_one(null, val);

:

recurse(startingPlace, otherFunc, function (recError, results) {
    if (recErr) {
         // treat the error from recursion
         return callback(); // important: use return, otherwise you will keep on executing whatever is there after the if part when the callback ends ;)
    }

    // No problems/errors
    console.log ("Recursion finished with results " + results);
    callback();   // writing down `return callback();` is not a bad habit when you want to stop execution there and actually call the callback()
});

, , get:

function myGet (a, callback) {
    request.get(a, function (error, response, body) {
        var nextPlace = JSON.parse(body).place;
        return callback(null, nextPlace); // null for no errors, and return the nextPlace to async
    });
}

var recurse = function(startingPlace, callback2) {
    request.get({
        url: 'bbb'
    }, function(error1, response1, body1) {
        // store body somewhere outside these funtions
        // make second request
        request.get({
            url: 'ccc'
        }, function(error2, response2, body2) {
            var anArray = JSON.parse(body2).stuff;
            if (anArray) {
                // The function that you want to call for each element of the array is `get`.
                // So, prepare these calls, but you also need to pass different arguments
                // and this is where `bind` comes into the picture and the link that I gave earlier.
                var theParallelCalls = [];
                for (var i = 0; i < anArray.length; i++) {
                    theParallelCalls.push(myGet.bind(null, {url: 'ddd'})); // Here, during the execution, parallel will pass its own callback as third argument of `myGet`; this is why we have callback and callback2 in the code
                }
                // Now perform the parallel calls:
                async.parallel(theParallelCalls, function (error3, results) {
                    // All the parallel calls have returned
                    for (var i = 0; i < results.length; i++) {
                        var nextPlace = results[i];
                        recurse(nextPlace, callback2);
                    }
                });
            } else {
                return callback2(null);
            }
        });
    });
};

, , get 'bbb' get 'ccc'. , , .

Answer 2

, , - , , .

callback (.. recurse start), , .

, :

get_all_pages(callback, page) {
    page = page || 1;
    request.get({
        url: "http://example.com/getPage.php",
        data: { page_number: 1 },
        success: function (data) {
           if (data.is_last_page) {
               // We are at the end so we call the callback
               callback(page);
           } else {
               // We are not at the end so we recurse
               get_all_pages(callback, page + 1);
           }
        }
    }
}

function show_page_count(data) {
    alert(data);
}

get_all_pages(show_page_count);

Answer 3

, caolan/async. async.waterfall. , , - .

:

async.waterfall([
    function(cb) {
        request.get({
            url: 'aaa.com'
        }, function(err, res, body) {
            if(err) {
                return cb(err);
            }

            cb(null, JSON.parse(body).id);
        });
    },
    function(id, cb) {
        // do that otherFunc now
        // ...
        cb(); // remember to pass result here
    }
], function (err, result) {
   // do something with possible error and result now
});

Answer 4

, :

var start = function(callback) {
  request.get({
    url: 'aaa.com'
  }, function (error, response, body) {
    var startingPlace = JSON.parse(body).id;
    recurse(startingPlace, otherFunc);
    // Call output function AFTER recursion has completed
    callback();
  });
};

, .

, .

var start = function(callback) {
  request.get({
    url: 'aaa.com'
  }, function (error, response, body) {
    var startingPlace = JSON.parse(body).id;
    recurse(startingPlace, otherFunc, callback);
  });
};

Answer 5

:

var udpate = function (callback){
    //Do stuff
    callback(null);
}

function doUpdate() {
    update(updateDone)
}

function updateDone(err) {
    if (err)
        throw err;
    else
        doUpdate()
}

doUpdate();