observ.js

module.exports = Observable

function Observable(value) {
    var listeners = []
    value = value === undefined ? null : value

    observable.set = function (v) {
        value = v
        listeners.forEach(function (f) {
            return f(v)
        })
    }
    observable.listen = function (listener) {
        listeners.push(listener)

        return function remove() {
            listeners.splice(listeners.indexOf(listener), 1)
        }
    }

    return observable

    function observable(listener) {
        if (!listener) {
            return value
        }

        listeners.push(listener)
        listener(value)

        return function remove() {
            listeners.splice(listeners.indexOf(listener), 1)
        }
    }
}