selectListOfMostRecent

selectListOfMostRecent.js

var Rx = require('rx');

(function(exports) {

    function shiftLeft(list, value) {
        var result = list.slice(1);
        result.push(value);
        return result;
    }

    exports.selectListOfMostRecent = function selectListOfMostRecent(count, defaultValue) {
        count = Math.max(count, 1);
        var listOfValues = [];
        for (var idx = 0; idx < count; ++idx) {
            listOfValues[idx] = defaultValue;
        }
        return this.scan(listOfValues, shiftLeft);
    };

})(Rx.Observable.prototype);

function print() { return console.log.apply(console, arguments); }
function increment(value) { return ++value; }

Rx.Observable
    .interval(800)
    .scan(0, increment)
    .selectListOfMostRecent(4, 0)
    .subscribe(print);