closures posts

Javascript: Closures, Objects, and Timeouts

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I recently discussed two issues with closures in Javascript. I continued to improve my menu script by fixing some IE issues, then I moved everything into objects. By moving to objects, I ran into another issue that was basically a combination of the two issues discussed in the previous article. I did not immediately realize this however, for whatever reason, and spent some time figuring it out.

The issue was with setting a timeout that calls a method of the object the timeout is set within. The method is attached to “this”, but “this” changes scope in a “setTimeout” call (“this” becomes “window”). To pass the appropriate “this”, we create a variable pointing to this and then pass it to an anonymous function that returns another anonymous function which uses are pointer to “this” to call the desired method. Since my “setInterval” was attached to an event listener, it also had to be placed inside of the double anonymous function. It looked something like this:

class.prototype.functionName = function(argElement){
    var fncThis = this;
    var callback = function(fncThis){
        return function(){
            fncThis.timeout = setTimeout(function(fncThis){ return function(){fncThis.classMethod(); };}(fncThis) ,750);
        };
    }(fncThis);
    argElement.addListenerEvent("touch", callbackMouseout, false);
}

This was the only major issue I had putting my code into objects. The last time I had tried something similar, I had given up dealing with the timeouts, but this time I had more experience. Now I have nice, reusable OO classes for suckerfish menus, etc.

Javascript: Closures, Scope, and Arrays

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Closures are used quite frequently in Javascript for tasks such as adding event listeners or setting timeouts on function calls. Closures are where a function is passed as a parameter to a function call from another function, and variables from the calling function must be used inside the parameter function. Dealing with scope in closures can be difficult, and I’ve spent a lot of time figuring issues with them out.

An early issue I ran into with scope, and a common one, is the loss of scope of the “this” keyword in the closed function. For example, you might want to do a setInterval that references the object that created it. To do so, you can simply create a variable pointing to “this” and then use that in the closed function, like:

class.prototype.thefunction = function(){
    var fncThis = this;
    setInterval(function(){ fncThis.doSomething(); }, 1000);
}

This is also a common problem with event listeners, where “this” might be hoped to point to the element the listener is related to, but doesn’t.

Recently, I ran into a closure problem while revamping the menu script we use at Cogneato for suckerfish menus from the old MM functions to something more capable.

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