Function composition

compose.js

var slice = [].slice

function compose() {
    var context = {}

    return slice.call(arguments).reduceRight(combineFunctions)

    function combineFunctions(memo, current) {
        return applyInOrder
        
        function applyInOrder() {
            var result = current.apply(context, arguments)
            return memo.call(context, result)
        }
    }
}​

function composeAsync() {
    var context = {}

    return slice.call(arguments).reduce(combineFunctions)

    function combineFunctions(memo, outer) {
        return applyInOrder

        function applyInOrder() {
            var inner = arguments[arguments.length - 1],
                arguments[arguments.length - 1] = applyOuter

            memo.apply(context, arguments)

            function applyOuter() {
                var args = [].slice.call(arguments).concat(inner)
                outer.apply(context, args)
            }
        }
    }
}​

test.js

var composedAsync = composeAsync(
    function(a, cb) { return cb(a * 2) },
    function(b, cb) { return cb(b * 3) },
    function(c, cb) { return cb(c * 4) }
)

composedAsync(4, console.log)

var composed = compose(
    function(a) { return a * 2 },
    function(b) { return b * 3 },
    function(c) { return c * 4 }
)

console.log(composed(4))