first.js

var first = (function() {
    var chainNames = ["then", "andThen", "next"];
    var endChainNames = ["finally", "andFinally", "last"];
    var chain = function(fn) {
        var f1 = function(g) {
            var func = function() {return g.call(this, fn.apply(this, arguments));};
            chain(func);
            return func;
        };
        var f2 = function(g) {
            return function() {return g.call(this, fn.apply(this, arguments));};
        };
        chainNames.forEach(function(name) {fn[name] = f1;});
        endChainNames.forEach(function(name) {fn[name] = f2;});
    };
    return function(f) {
        var fn = function() {
            return f.apply(this, arguments);
        };
        chain(fn);
        return fn;
    };
}());

// Ex: var f = first(add1).then(mult2).andThen(square).finally(negate);  f(3) = -((3 + 1) * 2)^2 = -64;
// TODO?: make aliases configurable properties of `first` function?

firstTests.js

//------------------------------------------------------------------------------
var add1 = function(x) {return x + 1;};
var mult2 = function(x) {return x * 2;};
var square = function(x) {return x * x;};
var negate = function(x) {return -x;};
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
console.assert(typeof first == "function", "first should be a function");
console.assert(typeof first(add1).then == "function", "first should add `then` and related functions to a function");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
var f = first(add1);
console.assert(3 == f(2), "first should not actually change behavior of existing function");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2);
console.assert(6 == f(2), "initial composition should work appropriately");
f = first(mult2).then(add1);
console.assert(5 == f(2), "initial composition should work appropriately: order matters");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2).then(square).then(negate);
console.assert(-36 == f(2), "multiple composition should work appropriately");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2).andThen(square).next(negate);
console.assert(-36 == f(2), "composition name aliases should be available");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2).andThen(square).finally(negate);
console.assert(-36 == f(2), "composition name aliases should include finalizer");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2).andThen(square).finally(negate);
console.assert(typeof f.then == "undefined", "finalized functions should not have chainable methods");
console.assert(typeof f.finally == "undefined", "finalized functions should not have finalizer methods");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
f = first(add1).then(mult2).andThen(square).andFinally(negate);
console.assert(-36 == f(2), "finalizers should also have aliases such as `andFinally`");
console.assert(typeof f.then == "undefined", "finalized functions should not have chainable methods");
f = first(add1).then(mult2).andThen(square).last(negate);
console.assert(-36 == f(2), "finalizers should also have aliases such as `last`");
console.assert(typeof f.last == "undefined", "finalized functions should not have finalizer methods");
//------------------------------------------------------------------------------


//------------------------------------------------------------------------------
var a = function() {return this.foo;};
var b = function(x) {return x + " " + this.bar;};
var obj = {
    foo: "Hello",
    bar: "world",
    f: first(a).then(b)
};
console.assert("Hello world" === obj.f(), "context is maintained when composed functions are used as methods");
//------------------------------------------------------------------------------