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Using Firefox, working on a Firefox extension, I continually get a javascript warning:
reference to undefined property mySidebar.context.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?
668
The undefined property indicates that a variable has not been assigned a value, or not declared at all.
Simply put, undefined means a variable has been declared but has not yet been assigned a value. undefined is a type by itself (undefined). Unassigned variables are initialized by JavaScript with a default value of undefined.
You will get undefined value when you call a non-existent property or method of an object. In the above example, a function Sum does not return any result but still we try to assign its resulted value to a variable. So in this case, result will be undefined.
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) {
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