Completely promise-based queue that works with functions returning promises and normal values.

queue.js

// Completely promise-based queue that works with functions returning promises and normal values.

/*
function doSomething() {
    return new Promise(y => setTimeout(() => y(), 3000));
}

// Using the singleton
for (let i = 0; i < 10; i++) Q(() => doSomething()).then(() => console.log(i));

// As an object (multiple queues)
var q = new Q();
for (let i = 0; i < 10; i++) q(() => doSomething()).then(() => console.log(i));
*/

function Q(fn, ...args) {
    // Call as constructor
    if (this.constructor === Q && typeof this.functions === "undefined") {
        const obj = { functions: [], paused: true, is: Q };
        obj.work = Q.work.bind(obj);
        return Q.bind(obj);
    } else if (typeof this.is === "undefined" || this.is !== Q) {
        return Q._singleton(fn, ...args);
    }   

    return new Promise(function(resolve, reject) {
        this.functions.push([fn, args, resolve, reject]);
        if (this.paused) this.work();
    }.bind(this));
}
Q.work = function work() {
    if (this.functions.length) {
        this.paused = false;
        const first = this.functions.shift();
        Promise.resolve(first[0](...first[1]))
        .then(function(resolve, value) {
            resolve(value);
            this.work();
        }.bind(this, first[2]))
        .catch(function(reject, value) {
            reject(value);
            this.work();
        }.bind(this, first[3]))
    } else {
        this.paused = true;
    }
}
Q._singleton = new Q();