Analog to Function constructor for building closures

closure.js

(function() {

    var hasOwnProperty = Object.prototype.hasOwnProperty;

    var Function = hasOwnProperty.constructor;

    function isIdentifier(s) {
        return /[a-zA-Z_$][a-zA-Z_$0-9]*/.test(s);
    }

    Closure = function Closure(args, body, env) {
        var closedVars = [], closedVals = [];

        for (var key in env) {
            if (!hasOwnProperty.call(env, key))
                continue;

            if (!isIdentifier(key))
                throw new Error("invalid variable name: " + key);

            closedVars.push(key);
            closedVals.push(env[key]);
        }

        // validate the syntax of the body as a FunctionBody
        (new Function(body));

        // validate the syntax of the formal parameters as FormalParameters
        Function.apply(null, args);

        // declare closed variables initialized from an array bound to `this`
        var closeVars = "var " + closedVars.map(function(x, i) {
            return x + " = this[" + i + "]";
        }).join(",") + ";";

        // return the function that closes over the variables
        var returnClosure = "return function(" + args.join(",") + "){" + body + "};";

        closedVals.maker = new Function(closeVars + returnClosure);

        return closedVals.maker();
    };

    Closure.prototype = Function.prototype;

})();

var f = new Closure([], "return x + y", { x: 1, y: 77 });
console.log(f()); // 78

var g = new Closure(["z"], "return x + y + z", { x: 1, y: 77 });
console.log(g(13)); // 91