Question

I’m trying to create a Postbox addon (Postbox is a mail client based on Thunderbird) and I’m having a small problem. I’m far from being a Javascript expert and I can’t understand the problem behind this…

I’m trying to extend some object function from the Postbox code. The code is quite big, so I tried to create a small example to demonstrate my problem. The following code is a sample of the original Postbox code structure:

FolderTreeView.prototype = {

    init: function FTV__init() {
        alert("init");
    },

    _ensureValidRow: function FTV__ensureValidRow(aRow) {
        alert("_ensureValidRow");
    },

    getCellProperties: function FTV_getCellProperties(aRow, aColumn, aProperties) {
        this._ensureValidRow(aRow);
    }

}

function FolderTreeView() {
    this._tree = null;
    this.init();
}

var gFolderView = new FolderTreeView();

I cannot change this code cause when Postbox gets updated, the code would revert to the original source and it would be a pain to maintain that.

The following is my own code, trying to extend the getCellProperties function:

MyExtension = {

    init: function() {
        MyExtension.FolderIcon.load();
    },

    FolderIcon: {
        load: function() {
            var oGetCellProperties = gFolderView.getCellProperties;

            gFolderView.getCellProperties = function FTV_getCellProperties(aRow, aColumn, aProperties) {
                oGetCellProperties(aRow, aColumn, aProperties);
            }

            gFolderView.getCellProperties(null, null, null);
        }
    }

}

Now, oGetCellProperties is calling the original function, which in turn tries to call this._ensureValidRow but it fails. The error console reports that:

this._ensureValidRow is not a function

Answer 1

a.b() sets the this value inside b to a. Saving it does not:

a.b(); // sets `this` value inside `b` to `a`

var c = a.b;

c(); // does *not* set `this` value inside `b` to `a`

So where you have:

var oGetCellProperties = gFolderView.getCellProperties;
// gFolderView is lost as `this` value

You rather want .bind. .bind is only available in newer browsers, but there are shims for older browsers.

var oGetCellProperties = gFolderView.getCellProperties.bind(gFolderView);
// force `this` value

Or you can set the this value each time you call, using .call:

//                      `this`       argument1, ...
oGetCellProperties.call(gFolderView, aRow,      aColumn, aProperties);