fingerscrossed.js

// Not a serious or proper implementation of promises.
// I haven't even bothered to look at the JS promises spec, and I
// know it's missing error handling at the least.
// Seriously, don't use this: it's just the most straightforward
// code to run the benchmark below, and nothing else.

var agenda = [];

function eventLoop() {
    var front = 0;
    while (front < agenda.length)
        agenda[front++]();
    agenda = [];
}

function defer() {
    var value;
    var watchers = [];           // (null after resolution)
    var promise = {
        then: function(f) {
            var deferred = defer(), resolve = deferred.resolve;
            var event = function() { resolve(f(value)); };
            if (watchers === null)
                agenda.push(event);
            else
                watchers.push(event);
            return deferred.promise;
        }
    };
    var resolve = function(resolution) {
        if (watchers === null)
            throw new Error("Multiple assignment to promise. Old: "
                            + value + " New: " + resolution);
        value = resolution;
        watchers.forEach(function(watcher) { agenda.push(watcher); });
        watchers = null;
    };
    return {
        promise: promise,
        resolve: resolve
    };
}

// From http://swannodette.github.io/2013/08/23/make-no-promises/
function benchmark() {
    var first = defer(), last = first.promise;
    for (var i = 0; i < 100000; ++i)
        last = last.then(function(val) { return val + 1; });

    var start = Date.now();
    first.resolve(0);
    last.then(function(val) {
        console.log("elapsed ms", Date.now()-start);
    });

    eventLoop();
}