Does undefined check ever throw an error in JavaScript without typeof in IE6?

I don't have an access to IE6 right now, so, I am asking here.

If I have a comparison such as:

// imagine App being declared as an obj somewhere...
if (App.errorLog === undefined) {
    App.errorLog = [];
}

Would that code throw an error in IE6 if the property was never declared or defined anywhere? It seems to work fine in other browsers even in IE7. I just think that I had some problems with IE6 while a back and used typeof to solve those problems, but I 开发者_C百科am not sure.

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Properties that were not set will simply return the value of undefined. Only undeclared variables will actually raise a ReferenceError, this is where you then have to use typeof foo === 'undefined'.

So yes, your code will work.

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works fine in IE6 as you have it

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The typeof trick resolves issues where the global undefined has been overwritten, or where a variable has not been declared.

In your case, you're just checking for the existence of a property in the App object. As long as App exists, there shouldn't be a declaration issue.

If there's a chance that some code will overwrite undefined, then you need typeof.

Another way to test for a property is:

if ( !('errorLog' in App) ) {
    App.errorLog = [];
}

This will also look at members of the prototype, and so is equivalent to checking for undefined and it gets around the issue of undefined being overwritten.

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i believe it would not throw an exception but simply it would go out of the if condition. also i have not heard of undefined as value. if you are trying to say that if the App.errorlog is not set then the code should be App.errorlog == null.