api.js

var events = require('events');
var callA = function (callback) {
    setTimeout(function () {
        callback({name: "a", data: "I am result A"});
    }, Math.round(Math.random() * 300));
};

var callB = function (callback) {
    setTimeout(function () {
        callback({name: "b", data: Math.round(Math.random() * 300) % 2 === 0});
    }, Math.round(Math.random() * 300));
};

var callC = function (callback) {
    setTimeout(function () {
        callback({name: "c", data: "I am result C"});
    }, Math.round(Math.random() * 300));
};

var api = function (callback) {
    var result = {}, counter = 0, timer;
    var emitter = new events.EventEmitter();
    var done = function () {
        clearTimeout(timer);
        emitter.removeAllListeners();
        if (result.hasOwnProperty("b") && result.b) {
            callC(function (c) {
                result[c.name] = c.data;
                callback(result);
            });
        } else {
            callback(result);
        }
    };
    var emit = function (ret) {
        emitter.emit('got', ret);
    };
    emitter.on('got', function (ret) {
        result[ret.name] = ret.data;
        counter++;
        if (counter === 2) {
            done();
        }
    });
    callA(emit);
    callB(emit);
    timer = setTimeout(function () {
        if (!result.hasOwnProperty('a')) {
            emitter.on('got', function (ret) {
                if (ret.name === 'a') {
                    done();
                }
            });
        } else {
            done();
        }
    }, 200);
};