WinJS.Class.define() - refer to member function in constructor

Question

I have a WinJS class defined as follows and would like to use a member function in the constructor:

WinJS.Class.define(
    function() {
        setInterval(myMemberFunction, 100);
    },
    { // Member variables
        myMemberFunction: function() {
            // Do something
        }
    });

Unfortunately it looks like I can't resolve member functions in that manner in the constructor. This code all lives in a module so I could move myMemberFunction up the hierarchy and access it from the constructor, however the drawback is that "this" would no longer refer to the instance of my WinJS class. What's the recommended method for accessing instance members in a WinJS from the constructor?

Answer 1

Generally speaking, you refer to any method or property using "this," as in this.myProperty. In the case of event handlers, you need to make sure that the "this" that you see inside the handler is the instance "this". That's the purpose of the bind method of a function object. So you do this:

setInterval(this.myMemberFunction.bind(this), 100);

This makes sure that you bind the right "this" instance to the callback. Because I've seen this question pop up frequently (use of .bind is all over the Windows SDK samples), I wrote about this in more detail on http://www.kraigbrockschmidt.com/2012/11/28/purpose-function-dot-bind/.

Just to note, this is pure JavaScript; nothing particular to WinJS or Windows Store apps.