// A promise implementation that enables sequencing and flattening of an arbitrary
// number of asyncronous functions, and allows a value to be passed through.

function first() {
    return promise(function(resolve, val) {
        setTimeout(function() {
            console.log("first", val);
            resolve(2);
        }, 50);
    });
}

first().then(function(resolve, val) {
    setTimeout(function() {
        console.log("second", val);
        resolve(val+1);
    }, 110);
}).then(function(resolve, val) {
    setTimeout(function() {
        console.log("third", val);
        resolve((val+1)*10);
    }, 60);
}).then(function(resolve, val) {
    setTimeout(function() {
        console.log("fourth", val);
        resolve();
    }, 40);
}).end();

console.log("zeroth");

// The code will output:
// zeroth
// first initVal
// second 2
// third 3
// fourth 40

function promise(f) {
    function callFns(arr, val) {                // Call functions in order and pass the val argument
        if (arr.length == 1) {
           return arr[0](function() {}, val);
        } else {
           arr[0](function(n) {                 // n is the resolve argument
               callFns(arr.slice(1), n);
           }, val);
        }
    }
    var fns = [f];                              // List of functions to call
    var res = {
        then: function(next) {
            fns.push(next);
            return res;                         // Recursively enable an arbitrary number of then-calls
        },
        end: function() {
            callFns(fns, "initVal");            // Pass all functions to be called together
                                                // with 'val' for the first of them.
        }
    };
    return res;
}