javascript reference to undefined property

Using Firefox, working on a Firefox extension, I continually get a javascript warning:

reference to undefined property mySidebar.co开发者_开发问答ntext.netProgress

I have tried multiple ways of testing the value:

if (mySidebar.context.netProgress === undefined) {

And

if (typeof mySidebar.context.netProgress == "undefined") {

And

if (!mySidebar.context.netProgress) {

And

if (mySidebar.context.netProgress == undefined) {

However the error console in Firefox continues to give me the warning on the same line every time, the line in question is the line that I posted the code from above. The actual check for the value is causing the warning.

I also put an alert to check the value of mySidebar.context, which is always an object, so it is not from the parent that I'm getting the warning.

Any ideas?

As Swingley said, you can use Object.prototype.hasOwnProperty() to check for existence of a direct property on an object. This won't work for properties inherited from the prototype chain, however. For both situations, inherited and direct, you can use the in operator:

if ("netProgress" in mySidebar.context) {

Try object.hasOwnProperty()

if (mySidebar.context.hasOwnProperty("netProgress")) {