PromisePool.js

//{{ PromisePool
/* Usage:
 var promise_pool = new PromisePool([concurrency_limit]);
    * concurrency_limit: n of concurrency promices executions
    generate a PromisePool

 promise_pool.push(function, args);
    * function: the function to generate promise object
    * args: the arguments array of function
    push a promise object into PromisePool, but the real Promise object has *NOT* been generated yet
 
 promise_pool.start();
    generate and execute all Promises in pool base on the concurrency_limit
    return a Promise object resolved when all Promises object in pool are resolved/rejected

 */

var PromisePool = function(limit) {
    this.queue = [];
    this.limit = (limit > 0) ? limit: 10;
};
PromisePool.prototype = {
    constructor: PromisePool,
    _dequeue: function() {
        var that = this;
        var group = that.queue.splice(0, that.limit);
        console.log(group.length, 'promises for process');
        group.forEach(function(p, i){
            group[i] = p[0].apply(this, p[1]);
        });
        return Promise.all(group);
    },
    push: function(fn, args){
        this.queue.push([fn, args]);
    },
    start: function(){
        var that = this;
        return new Promise(function(resolve, reject){
            var results = [];
            (function process(){
                that._dequeue().then(function(r){
                    console.log(r.length, 'done');
                    results.push.apply(results, r);
                    console.log(results.length, 'results merge done');
                    if(that.queue.length != 0) {
                        console.log(that.queue.length, 'remains');
                        process();
                    } else {
                        console.log(results.length, 'all done');
                        resolve(results);
                    }
                });
            })();
        });
    }
};
////}}

//{{ Scenario 1
    // Old Scheme
    var promise_list = [];
    promise_list.push(new Promise(function(resolve, reject){
        // promise 1 
    }));
    promise_list.push(new Promise(function(resolve, reject){
        // promise 2 
    }));
    promise_list.push(new Promise(function(resolve, reject){
        // promise 3 
    }));
    Promise.all(promise_list).then(function(results){
        console.log('all done');
    });


    // New Scheme
    var promise_list = new PromisePool(10);
    promise_list.push(fnToGeneratePromise1, [/*args of fnToGeneratePromise1*/]);
    promise_list.push(fnToGeneratePromise2, [/*args of fnToGeneratePromise2*/]);
    promise_list.push(fnToGeneratePromise3, [/*args of fnToGeneratePromise3*/]);

    promise_list.start().then(function(results){
        console.log('all done');
    });
////}}

//{{ Scenario 2

    /* jQuery included... */
    var getUserData = function(user_id){
        return new Promise(resolve, reject) {
            $.get('getUserData.php?user_id='+user_id).then(function(rsp){
                resolve(rsp);
            });
        };
    };

    // Old Scheme
    var promise_list = [];
    promise_list.push(getUserData('user1'));
    promise_list.push(getUserData('user2'));
    // :
    // Skip lots of promise_list.push
    // :
    promise_list.push(getUserData('user100'));
    Promise.all(promise_list).then(function(results){
        console.log('all user data loaded');
    });

    // New Scheme
    var promise_list = new PromisePool(10);
    promise_list.push(getUserData, ['user1']);
    promise_list.push(getUserData, ['user2']);
    // :
    // Skip lots of promise_list.push
    // :
    promise_list.push(getUserData, ['user100']);
    promise_list.start().then(function(results){
        console.log('all user data loaded');
    });
//}}