JavaScript immutable List and Lens implementation

lens.js

var List = (function() {
    function List(src) {
        this._list = null
        this._length = 0

        if(src instanceof List) {
            this._list = src._list
            this._length = src._length
        }
    }

    List.fromArray = function(src) {
        var list = new List()
        for (var i = src.length - 1; i >= 0; i--) {
            list = list.push(src[i])
        }
        return list;
    }

    List.prototype = {
        head: function () {
            return this._list === null ? null : this._list.head;
        },

        tail: function () {
            return this._list === null ? null : this._list.tail;
        },

        push: function (value) {
            var result = new List(this)
            result._list = { head: value, tail: this._list === null ? null : this }
            result._length = result._length + 1
            return result
        },

        length: function () {
            return this._length
        },

        map: function (fn) {
            var arr = []
            var list = this
            while(list !== null) {
                arr.push(fn(list.head()))
                list = list.tail()
            }
            return List.fromArray(arr)
        }
    }

    return List
})();

var Lens = (function() {
    function copy(src) {
        if (src === null || typeof src !== "object") return obj;

        if (src instanceof Date) {
            var date = new Date()
            date.setTime(src.getTime())
            return date
        }

        // Lists are immutable
        if (src instanceof List) return src

        if (src instanceof Array) return List.fromArray(src)

        if (src instanceof Object) {
            var obj = {};
            
            for (var attribute in src) {
                if (src.hasOwnProperty(attribute)) {
                    obj[attribute] = src[attribute]
                }
            }

            return obj
        }
    }

    function Lens(fieldName) {
        this._fieldName = fieldName;
    }

    Lens.copyObject = copy

    Lens.prototype = {
        get: function(model) {
            if (!model) {
                return this.get
            }

            return model[this._fieldName]
        },

        set: function(value, model) {
            var self = this;

            if (!model) {
                return function(m) {
                    return self.set(value, m)
                }
            }

            var result = copy(model)
            result[this._fieldName] = value
            return result
        },

        modify: function(fn, model) {
            var self = this;

            if (!model) {
                return function(m) {
                    return self.modify(fn, m)
                }
            }

            return this.set(fn(this.get(model)), model)
        },

        compose: function(lens) {
            return new ComposedLens(this, lens)
        }
    }

    function ComposedLens(lens1, lens2) {
        this._lens1 = lens1;
        this._lens2 = lens2;
    }

    ComposedLens.prototype = {
        get: function(model) {
            if (!model) {
                return this.get
            }

            return this._lens2.get(this._lens1.get(model))
        },

        set: function(value, model) {
            var self = this
            if (!model) {
                return function(m) {
                    return self.set(value, m)
                }
            }

            return this._lens1.modify(this._lens2.set(value), model)
        },

        modify: Lens.prototype.modify,

        compose: Lens.prototype.compose
    }

    return Lens
})();

// some simple tests
var sqr = function(x) { return x * x }
var l0 = List.fromArray([10, 11])
var l1 = l0.map(sqr)


// need to figure out how to push into a list (i.e. author could have a list of cars)
var car = {
    color: "red"
}

var author = {
    name: "bob",
    car: car
}

var book = {
    title: "blarg",
    author: author
}

var bookTitle = new Lens("title")
var bookAuthor = new Lens("author")
var authorName = new Lens("name")
var authorCar = new Lens("car")
var carColor = new Lens("color")

var bookAuthorName = bookAuthor.compose(authorName)
var bookAuthorCarColor = bookAuthor.compose(authorCar).compose(carColor);

var title = bookTitle.get(book)

var bookb = bookTitle.set("zombies", book)
var bookc = bookAuthorName.set("joe", book)
var bookd = bookAuthorCarColor.set("blue", book)