Using the "thisObject" parameter in Array.filter, .every, .some, .forEach, .map

I have not found any documentation and tutorials for using the thisObject parameter within the Array class in ActionScript 3. It seems that there is a gap in knowledge of how to use this, and it could save some people a lot of coding. I think it comes from Adobe copying Javascript docs for Array, essentially (which does not provide much help in using thisObject, either).

thisObject replaces what is considered the “this” scope. If you are working in some class, and type this.someVar, you are accessing the class-level variable called someVar. This is pretty simple.

Within an Array.filer() call, though, you probably want to have the scope be something else. Often, you will see code with hard-coded values used for filter, every, some, etc. Like

array.filter(someCallbackFunction);
[...]
public function someCallbackFunction(element:*, index:int, arr:Array):Boolean
{
    element.isActive = true;
}

In someCallbackFunction, the value “true” is hard-coded. What if you want to test against “false”? That would require another function. Not very good OOP.

If the hard-coded values could be a variable instead, that would make things much easier to understand. thisObject supplies a context for using variables.

If you have an array of objects of type SampleClass, and you want to filter according to some variable in the class, set thisObject to the value to compare. For example:

var testVal:String = "something";
array.filter(someCallbackFunction, testVal);
[...]
public function someCallbackFunction(element:*, index:int, arr:Array):Boolean
{
        element.isActive = this;
}

Now someCallbackFunction uses testVal as the local context; this actually equals testVal, so you can just use this to reference the data, as above.

Here is a working method example:

public function testArray()
{
    var arr:Array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
    var filterValArr:Array = [4, 8];

    var filtARr:Array;
    var val:int = filterValArr.length;
    for (var i:int = 0; i < val; i++)
    {
        filtARr = arr.filter(
            function(item:*, index:int, array:Array):Boolean
            {
                return item > this;
            },
            filterValArr[i]
        );
        trace("filtARr test " + i + " = " + filtARr);
    }
}

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Adobe, AS3.0, Flash, javascript

Actionscript 3.0, Adobe, array, AS3.0, each, every, filter, flash, forEach, map, some, thisObject

2010/04/25

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