demonstration of function composition using Fortinbras components

gistfile1.js

/** 
 * Standalone demonstration of function composition using Fortinbras components
 * Paste into a browser console and run. (Bonus demo: safe use of ASI)
 * @author Noah Feldman
 */

// basic compose looks like: compose(f, g) (x) = f(g(x))
// or: apply argFunc to the arguments, then apply func to the result
// mine does the same thing, but let's us supply extra arguments
function compose (func, argFunc, thisObj) {
    return function () { // call argFunc with as many of the arguments as it can use, then call func with the result plus any remaining arguments
        return func.apply(thisObj, concat.call([argFunc.apply(thisObj, arguments)], splice.call(arguments, argFunc.length, arguments.length)))
    }
}

var splice = Array.prototype.splice,
    concat = Array.prototype.concat,
    exists = (function (U) {
        return function (obj, prop) {
            var idx;
            if (prop === '' && obj)
                return true
            if (obj) {
                idx = prop.indexOf('.')
                if (idx > -1)
                    return (obj = obj[prop.slice(0, idx)]) === U ? false : exists(obj, prop.slice(++idx, prop.length))
                else 
                    return typeof obj[prop] != 'undefined' && obj[prop] !== U
            }
            return false
        }
    }()),
    isGetValue = function (a, b) {
        var nest, len
        if (exists(a, b)) {
            nest = b.split('.').reverse()
            len = nest.length
            while (--len)
                a = a[nest[len]]
            return a[nest[0]]
        }
        return false
    },

    propEql = compose(function (a, b) { return a === b }, isGetValue)
    
//  CONSOLE TEST
//  GIVEN
    var t = {foo: {bar: {baz: {say: 'hello'}}}}
//  THEN
    propEql(t, 'foo.bar.baz.say', 'hello') === true
//  BECAUSE 
//  We call isGetValue(t, 'foo.bar.baz.say'), then we apply the lambda to the
//  return value of isGetValue and 'hello'