mutex.js

var Qeueue = require("q/queue");
var semaphore = new Queue();
semaphore.put(); // once for one job at a time

function exclusive(method) {
    return function wrapped() {
        var self = this, args = Array.prototype.slice.call(arguments);
        return semaphore.get()
        .then(function () {
            return method.apply(self, args);
        }).finally(semaphore.put);
    };
};

var a = exclusive(a);
var b = exclusive(b);