index.js

var once = require('async.util.once');
var noop = require('async.util.noop');
var onlyOnce = require('async.util.onlyonce');
var keyIterator = require('async.util.keyiterator');

module.exports = function eachOfLimit(limit) {
    return function(obj, iterator, cb) {
        cb = once(cb || noop);
        obj = obj || [];
        var nextKey = keyIterator(obj);
        if (limit <= 0) {
            return cb(null);
        }
        var done = false;
        var running = 0;
        var errored = false;

        (function replenish() {
            if (done && running <= 0) {
                return cb(null);
            }

            while (running < limit && !errored) {
                var key = nextKey();
                if (key === null) {
                    done = true;
                    if (running <= 0) {
                        cb(null);
                    }
                    return;
                }
                running += 1;
                iterator(obj[key], key, onlyOnce(function(err) {
                    running -= 1;
                    if (err) {
                        cb(err);
                        errored = true;
                    } else {
                        replenish();
                    }
                }));
            }
        })();
    };
};