Promise based queue

box.js

function mkBox() {
    var box = {values: [], reads: []};
    
    box.enq = function(value) {
        box.values.push(value);
        if (box.reads.length > 0) {
            box.reads.shift().resolve(box.values.shift());
        }
        return box;
    };
    
    box.deq = function() {
        var d = Q.defer();
        if (box.values.length > 0 && box.reads.length == 0) {
            d.resolve(box.values.shift());
        } else {
            box.reads.push(d);
        }
        return d.promise;
    };
    
    return box;
}