Shoutbox

• You're trying to add elements to the timers array, but since you're using a string as key you're actually just setting an object property. Either you use numerical indexes and make proper use of the array, or you just make timers an object. Because you're not actually using any array-specific features, you can simply get away with an object here.
  
• Instead of parsing data out of the keys in OnEvent_Timer, you can just store objects as elements in timers and read the properties instead.
  
• You should check whether the timerId actually exists in timers in OnEvent_Timer. Sure, chances are slim that you'll ever get an invalid timer, but you never know.
  
• Try to reuse some of your code! setTimeout and setInterval are almost identical.

Javascript code:

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/* Somewhat private worker classes */
// Timer sets up a timer and exposes methods
var Timer = function(fCallback, nInterval) {
    this.Callback = function() {
        fCallback.call(this);
        this.Cancel();
    };
    this.Interval = nInterval;
    // Add to storage
    this.Index = Timer.Storage.length;
    Timer.Storage.push(this);
    // Identifier for timer events
    this.Id = 'Timer#'+this.Index;
    MsgPlus.AddTimer(this.Id, this.Interval);
};
// Storage array as static property
Timer.Storage = [];
// Instance methods
Timer.prototype = {
    Refresh : function() {
        Timers[this.Index] = this;
        MsgPlus.AddTimer(this.Id, this.Interval);
    },
    Cancel : function() {
        Timers[this.Index] = null;
        MsgPlus.CancelTimer(this.Id);
    }
};
 
// Interval is identical to Timer, except for its Callback property
var Interval = function(fCallback, nInterval) {
    // Call superclass constructor
    Timer.apply(this, arguments);
    // Callback refreshes timer instead of canceling it
    this.Callback = function() {
        fCallback.call(this);
        this.Refresh();
    };
};
Interval.prototype = Timer.prototype; // Prototypical inheritance! :O
 
// Timer event handling
function OnEvent_Timer(timerId) {
    if((m = /^Timer#(\d+)$/.exec(timerId)) !== null) {
        var nIndex = 1*m[1], timer = Timer.Storage[nIndex];
        if(timer && timer.Callback) {
            timer.Callback();
            return;
        }
    }
    // Other timers can be handled here...
}
 
function setTimeout(func, delay) {
    var timer = new Timer(func, delay);
    return timer.Index;
}
function setInterval(func, delay) {
    var interval = new Interval(func, delay);
    return interval.Index;
}
 
function clearTimeout(timerIndex) {
    var timer = Timer.Storage[timerIndex];
    if(timer && timer.Cancel) {
        timer.Cancel();
    }
}
var clearInterval = clearTimeout;

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