atom.js

(function() {

    function atom(val, options) {
        var watchers = {};
        var validator = options && options.validator || function () { return true; };

        function transition(next) {
            if (!validator(next)) {
                var err = new Error(next + " failed validation");
                err.name = "AssertionError";
                throw err;
            }

            var prev = val;
            val = next;

            Object.keys(watchers).forEach(function (k) {
                watchers[k](k, atom, prev, next);
            });
        }

        var atom = {
            addWatch: function (key, fn) {
                watchers[key] = fn;
            },

            removeWatch: function (key) {
                delete watchers[key];
            },

            swap: function (fn) {
                var args = [val].concat([].slice.call(arguments, 1));
                transition(fn.apply(null, args));
            },

            reset: function (v) {
                transition(v);
            },

            deref: function () {
                return val;
            },

            toString: function () {
                return "Atom(" + JSON.stringify(val) + ")";
            }
        };

        return atom;
    }

    function deref(ref) {
        return ref.deref();
    }

    this.createAtom = atom;
    this.atom = {
        create: atom,
        deref: deref
    };

}).call(this);