example.js

let {event, map, filter, accumulate, forEach} = require("./frp.js")

let {signal: time, feed} = event()

var encodedTime = map(time, x => new Date(x))

var oddSeconds = filter(encodedTime, time => time.getSeconds() % 2 == 1)

var threeLastTimesWithOddSeconds =
    accumulate(oddSeconds, [], (acc, time) => {
        if (acc.length > 2)
            acc.shift()
        acc.push(time)
        return acc
    })

forEach(threeLastTimesWithOddSeconds, array => {
    console.log(array)
})

console.log("Machinery is built, starting...")

for (var i = 0; i < 20; i++) {
    setTimeout(() => {
        feed(Date.now())
    }, i * 344)
}

frp.js

var event = () => {
    var signal = {subscriptions: (x) => null}
    var feed   = (x) => signal.subscriptions(x)
    return {signal, feed}
}

var subscribe = (signal, feed_transform) => {
    let {signal: result, feed} = event()
    var old = signal.subscriptions
    signal.subscriptions = (x) => {
        feed_transform(feed, x)
        old(x)
    }
    return result
}

var map = (signal, f) => 
    subscribe(signal, (feed, x) => 
        feed(f(x))
    )

var filter = (signal, pred) =>
    subscribe(signal, (feed, x) => {
        if (pred(x)) feed(x)
    })

var accumulate = (signal, zero, plus) => 
    subscribe(signal, (feed, x) => {
        feed(zero = plus(zero, x))
    })

var forEach = (signal, handler) => 
    subscribe(signal, (feed, x) => {
        handler(x)
    })

module.exports = {event, map, filter, accumulate, forEach}