feature.js

(function (global, pubsub, cache) {   
    function push(prop) {
        return function (val) {
            this[prop].push(val);
            return this;
        };
    }
    function fire(data) {
        var proceed = true;
        this.start.every(function (fn) {
            return !fn(data) && (proceed = false);
        });
        if (proceed) {
            this.stop.some(function (fn) {
                return fn(data) && !(proceed = false);
            });
        }
        if (proceed) {
            this.action.forEach(function (fn) {
                fn(data);
            });
        }
    }
    function Feature(description) {
        if (!(this instanceof Feature)) {
            return new Feature(description);
        }
        this.description = description;
        this.start = [];
        this.stop = [];
        this.action = [];
        this.fire = fire.bind(this);
        this.topic = null;
        cache[description] = this;
    }
    Feature.get = function (description) {
        return cache[description];
    };
    Feature.mung = function (prop, rename) {
        this.prototype[rename] = this.prototype[prop];
        return this;
    };
    Feature.prototype = {
        given: push("start"),
        unless: push("stop"),
        then: push("action"),
        until: function (topic) {
            var self = this;
            pubsub.subscribe(topic, function () {
                pubsub.unsubscribe(self.topic, self.fire);
            });
            return this;
        },
        when: function (topic) {
            this.topic = topic;
            pubsub.subscribe(topic, this.fire);
            return this;
        }
    };
    Feature.mung("given", "and")
        .mung("unless", "or");
    
    global.Feature = Feature;
}(this, pubsub, {}));

feature_example.js

Feature("run FeatA under the right conditions")
    .given(function (data) {return typeof data === "number";})
    .and(function (data) {return !!data;})
    .when("FeatA")
    .then(function(data){console.log("pass", data);})
    .unless(function (data) {return data === 1})
    .or(function (data) {return data%2 === 0})
    .until("stopFeatA");
    
pubsub.publish("FeatA", 3)   //pass 3
    .publish("FeatA", 1)     //nothing
    .publish("FeatA", 2)     //nothing
    .publish("stopFeatA")
    .publish("FeatA", 3)     //nothing

Feature.get("run FeatA under the right conditions").fire(3); //pass 3

feature_pubsub.js

(function (global, cache) {
    global.pubsub = {
        publish: function (topic, data) {
            (cache[topic] || []).forEach(function (sub) {
                sub(data);
            });
            return this;
        },
        subscribe: function (topic, fn) {
            (cache[topic] = cache[topic] || []).push(fn);
            return this;
        },
        unsubscribe: function (topic, fn) {
            (cache[topic] || []).some(function (sub, i, arr) {
                return sub === fn && !!(arr.splice(i, 1));
            });
            return this;
        }
    };
}(this, {}));