compose.js

(function (global, undef) {
    /**
     * memoizes functions with a single, primitive argument
     * @param f {Function}
     * @returns {Function}
     */
    function memoize(f) {
        var cache = (f.memoize = f.memoize || {});
        return function (arg) {
            return (arg in cache) ? cache[arg] : cache[arg] = f.call(this, arg);
        };
    }

    /**
     * create a function to compose n functions
     * @param n {Integer}
     * @returns {Function}
     */
    var c = memoize(function (n) {
        if (n < 1) {
            return new Function;
        }
        var i;
        var params = "";
        var vars = "\treturn function () {\n";
        var body = "\t\treturn _0;\n";
        for (i = 0; i < n; i += 1) {
            params += i ? (", fn" + i) : "fn" + i;
        }
        vars += "\t\tvar _" + --i + " = fn" + i + ".apply(this, arguments);\n";
        while (i--) {
            vars += "\t\tvar _" + i + " = fn" + i + ".call(this, _" + (i + 1) + ");\n"
        }
        body += "\t};";
        return Function(params, vars + body);
    });
    
    /**
     * Composes 1-n functions
     * @param {Function}
     * @returns {Function}
     */
    global.compose = function () {
        return c(arguments.length).apply(this, arguments);
    };
}(this));

compose_example.js

function slice(obj) {
    return Array.prototype.slice.call(obj, 0);
} 
function makeArray(obj) {
    if (obj === null || obj === undefined) {
        return [];
    }
    if (obj instanceof Array) {
        return obj;
    }
    if (obj instanceof Object && "length" in obj && "0" in obj) {
        return slice(obj);
    }
    return [obj];
}
var fixedArr = [1,2,3];
function fixed() {
    return fixedArr;
}
function makeUnary(arg) {
    if (arguments.length > 1) {
        return arguments;
    }
    return arg;
}
//compose slice and makeArray passing fixed as a parameter
console.assert(JSON.stringify(compose(slice, makeArray)(fixed())) === "[1,2,3]");
console.assert(compose(slice, makeArray)(fixed()) !== fixed());
//compose slice, makeArray, and fixed
console.assert(JSON.stringify(compose(slice, makeArray, fixed)()) === "[1,2,3]");
console.assert(compose(slice, makeArray, fixed)() !== fixed());
//compose slice, makeArray, and makeUnary so any signature will produce a list
var list = compose(slice, makeArray, makeUnary);
console.assert(JSON.stringify(list(1)) === "[1]");
console.assert(JSON.stringify(list(1,2)) === "[1,2]");
console.assert(JSON.stringify(list([1,2])) === "[1,2]");
console.assert(JSON.stringify(list({0:1,1:2,length:2})) === "[1,2]");