A way to enable Kefir streams to implement a timeout on the next event. Modified to improve timeout handling.

promiseNextBefore.js

var Kefir = require("kefir");

var period = 1000;
//var wait = 2 * period; //success condition
var wait = 0.5 * period; //failure condition

function promiseNextBefore(stream, timeout){
    return new Promise(function(resolve, reject){
        function handler(event){
            if(event.type==="value"){
                resolve(event.value);
            }
            else{
                reject(event);
            }
            unsubscribe();
        }
        var timeoutId = setTimeout(function(){
            if(handler){
                unsubscribe();
                reject("Promise of next event timed out after " + timeout);
            }
        }, timeout);
        function unsubscribe(){
            if(timeoutId){
                stream.offAny(handler);
                clearTimeout(timeoutId);
                timeoutId = null;
            }
        }
        stream.onAny(handler);
    });
}

var promise = promiseNextBefore(Kefir.interval(period, "Hello World"), wait).then(
    function(value){ console.log("Succeeded with " + value); },
    function(err){ console.log("Failed with " + err.toString()); }
);