I was working on some javascript stuff for a project and ran into this problem with setInterval(): for IE 6, calling setInterval() to an object’s method results in the wrong scope. In the object’s method, the alfamous **this** keyword now refers to **window**, instead of the conventional “this” as in “**this object**”. So let me give you some more details on this and a free, amazingly simple solution to this head-ache.
**SPOILER:**\\ the solution uses eval(). if you are allergic to eval(), don’t take my advice.\\ **END SPOILER**
===== The code =====
Let’s look at some code and we’ll have a little discusssion. Here’s the psuedo-code for a fictional TimeTicker object, which is supposed to automatically update a “clock” on the screen every second.
/* A. Constructor */
function TimeTicker()
{
/* A.1 - populate internal variable here, like hour, minute, second, etc. */
...
/* A.2 - now we set the timer to periodically update the time value every 999 miliseconds */
this.timer = setInterval( this.updateTime, 999 );
}
/* B. This function is to update the time value somewhere on the screen */
TimeTicker.prototype.updateTime = function ()
{
/* B.1 - compute new value */
...
this.hour = new_hour_value;
this.minute = new_minute_value;
this.second = new_second_value;
...
/* B.2 - now update the screen */
...
}
===== The problem =====
The above psuedo code will sadly **FAIL** in both IE and FF (not because it’s only psuedo-code!), as the ‘’setInterval()” invocation has **totally changed the scope** of the polled function. First of all, let’s look at ”**A.2**” from above. Calling ”this” within the ‘’setInterval()” will change the reference of ”this” to some other object (which I think is ”this” now refers to ”window”.) Hence calling ‘’setInterval( this.updateTime, 999 )” fails in both Firefox and IE 6, as it means that the ‘’setInterval()” will poll the ”window.updateTime()” function every 999 ms. That’s not what we want, and ”window” object could care less about our ”updateTime()” since we suddenly found ourselves in the wrong scope.
===== The Firefox’s Solution =====
For firefox, the fix is a small rewrite of how we would call the polled-function, updateTime(). The setInterval() function in Firefox is improved and we can now pass an extra parameter to it, and that’s exactly what we will do. We pass in the current object’s reference as ”this” to the a proxy function, which in turn makes a call to the updateTime(). I found this solution through [[http://www.klevo.sk/javascript/javascripts-settimeout-and-how-to-use-it-with-your-methods/|Klevo’s blog]], a web developer from Slovakia.
We will have to rewrite ”**A.2**” as follows:
/* A.2 */
this.timer = setInterval ( function( that ) { that.updateTime(); }, 999, this );
What we just did is we create a proxy function, whose only parameter is ”that”, a pointer to another object. Within this proxy function, we can make invoke all of “that”’s methods! Basically we migrate the scope of ”this” as a reference to the current ”TimerTicker” object into the polled proxy function via a pass-by-reference ”that” parameter. Notice the extra 3rd parameter of ‘’setInterval()”: we are passing ”this”, in this case, a reference to the current ”TimeTicker” object, as the only parameter of the newly created proxy function. Since now ”that” is equivalent to ”this”, we can call all methods of TimeTicker object! Beautiful!
===== The Internet Explorer’s Problem =====
While setInterval() in Firefox provide us a way to pass in extra parameters, IE’s version of ‘’setInterval()” is less flexible (and it also did drive me nuts!) Anyway, let’s talk about the mechanism of IE’s ‘’setInterval()”. According to [[http://msdn.microsoft.com/workshop/author/dhtml/reference/methods/setinterval.asp|MSDN’s manual on setInterval()]], the syntax of setInterval() is
”iTimerID” = ”window”.**setInterval(** ”vCode”, ”iMilliSeconds” [, sLanguage])
with ”vCode” is either a reference pointer or a “string that indicates the code to be executed when the specified interval has elapsed”. Since there’s no way we can pass an extra parameter to the polled-function, we cannot pass ”this” as a 3rd parameter of setInterval() as what we just did for Firefox.
If we rewrite the code as this.timer = setInterval ( this.updateTime(), 999);then 2 things are gonna happen: first,” this.updateTime()” is executed right away. It’s equivalent to a direct call to the ”updateTime()” function of the current object, ”TimeTicker”. As we are still in the scope of the current function, hence, ”this” refers to the current ”TimeTicker” object. Secondly, after the 999ms has elapsed, ‘’setInterval()”" will invoke ”this.updateTime()”, and now ”this” refers to ”window” as we are now out of the scope of the ”TimeTicker” object. And of course, ”window.updateTime()” is wrong. So what you see is that your script will work the first time (when this.updateTime() is executed in the correct scope), and then everything fails (when this.updateTime() is excuted by setInterval() in the wrong scope).
And If we write this.timer = setInterval ( this.updateTime, 999);(Notice the missing parenthesis). This will also fails as when this.updateTime is invoked, we are already in the wrong scope, the scope of ”window” object!
On the other hand, if we writethis.timer = setInterval ("this.updateTime()", 999);we are still in the wrong scope when setInterval() invoke the updateTime() method. ”this” still refers to ”window” when the updateTime() function is executed. Notice that we can include the parenthesis “( )” (and potentially other pass-by-value parameters since the method is refered to as a string and it’s not executed right away, but only when being invoiked by ‘’setInterval()”). However, we can’t pass a pointer to the current object in a string!
We’d like to pass a pass-by-reference pointer object to point to the current ”TimeTicker” object but no matter what we do, we cannot seem to do so. But be hold! Our salvation comes from the use of a glorifying global variable, and of course, a dash of ”eval()”, just enough to tame Internet Explorer.
===== The Internet Explorer’s Solution =====
I’d like to include the code for IE here, just read over and we will discuss later.
/* C. global scope object */
var globalScope = new Array();
/* A. Constructor */
function TimeTicker()
{
/* A.0 - An extra unique identifier for the current object */
this.uniqueId = some_text_or_numers;
/* A.1 - populate internal variable here, like hour, minute, second, etc. */
...
/* A.3 - setInterval Fix */
/* A.3.1 - for IE */
if( document.all )
{
/* A.3.1.1 - make a reference to the current object and saved in the global scope array
* to use later to correct the scope.
*/
globalScope[ this.uniqueId ] = this;
setInterval( 'ieIntervalHandler("' + this.uniqueId + '","updateTime")', 999 );
}
else
{
/* A.3.2 - Mozilla */
this.timer = setInterval ( function( that ) { that.updateTime(); }, 999, this );
}
}
And we add an extra function, ”ieIntervalHandler( **id**, **strFunc** )” to the script.
/* D. - Special IE setInterval handler function with scope corrected */
function ieIntervalHandler( id, strFunc )
{
/* D.1 - correct the scope then make the call */
var scope = globalScope[id];
eval( "scope." + strFunc + "()" );
}
All set? Here comes the explanations: First of all, we added a global associative array named globalScope (”**C**”). This associative array is used to store reference to a particle object based on that object’s unique identifier. Confused? Not a problem, just keep on reading.
Let’s read A.0, this is where we set the unique identifier for this ”TimeTicker” object. For this technique to work, each object **must** have a **unique identifier** since the unique identifier is used as the **key** of the global associative array.
Look at ”**A.3.1**”. Here we do a little browser-intelligence. If the browser doesn’t know ”document.all”, which is IE-specific, we use the improved version of ‘’setInterval()” as mentioned in the above Firefox’s solution section. If the browser is indeed IE, at ”**A.3.1.1**”, we store the reference to this ”TimeTicker” object as an element in the globalScope associative array. Then we cheerfully and confidently ‘’setInterval()” the ”ieIntervalHandler()” function (see ”**D.1**”). We do a pass-by-value the unique ID of the current ”TimeTicker” object as the first parameter of ”ieIntervalHandler()”, and ”updateTime” as the name of the method of the same ”TimeTicker” object.
Look at ”**D.1**”. Here we have a 2-line function. First, we get the reference to the particular TimeTicker object, which we already stored in the globalScope associative array back in ”**A.3.1.1**”. Since we are now in the scope of ieIntervalHandler(), our ‘’scope” variable is pointing to the ”TimeTicker” object. With the correct reference to the correct object, now we can make the call to the correct method with a quick, painless ”eval()”eval( "scope." + strFunc + "()" );
What did we just do? We use a global array to store the references so that later on we can get to the right object, even when ieIntervalHandler is invoked by ‘’setInterval()” and the scope now is all over the place — we could care less! Hence we now can solve the wrong scope issue within IE. And guess what, now we can have any object to poll a method of itself! How awesome is that?
===== Memory Issue =====
Since we need to keep polling a function, we need to be careful of not creating more and more objects or functions that the garbage collect will never be able to purge because of a hidden reference somewhere. If you do create a lot of objects that do self-polling using this global associative array technique, when destroying the object, you will probably have to set the element of the globalScope to ”**null**” also, so that there is no further reference to the destroyed object. Also, in ”**A.3.2**”, we do create a new function everytime we’d like to run the timer, this may be a potential memory leaking point on Firefox.
I’d like to leave the quest of battling memory-leak monster to somebody else, but just remeber to watch out for potential weak point where memory leak can happen, especially when your page will be opened for a prolonged preriod of time.
===== Keywords spam =====
This setInterval() and the scope problem is a pretty abstract and hard to describe problem. I just include some potential search keywords here so that other people can by chance find this page. Call it keyword spam, call it shameless-self advertising, I could care less, but if somebody find this helpful, then I call it success 
setInterval scope loosing, setTimeout, this setInterval wrong scope, javascript setInterval scope, global associative array scope correction, object oriented setInterval, self polling, this polling scope error, internet explorer setInterval fail, firefox setInterval fail, setInterval object.
===== Final remarks =====
Thank you for reading! I hope this helps. And I’d like to wish you a …
Merry Christmas and Happy New Year!
4:13AM, Dec 23, 2006.
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