proposal: change checks for undefined

[DanielRuf]

Description

Currently we have checks like val === undefined which is not correct in general
Correct would be typeof val === 'undefined' as this could fail on some systems.

Should we change this to use typeof?

Possible Solution

Use typeof check.

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[ncoden]

Nice idea. But just to be sure, could you provide some references or tell on which systems val === undefined would fail ?

[DanielRuf]

on which systems val === undefined would fail ?

It failed on a local setup with some Node versions with foundation-sites 6.4.3 (not sure if 6 or 8 or 10, was on Friday at work).

[ncoden]

https://stackoverflow.com/a/4725697/4317384

For undeclared variables, typeof foo will return the string literal "undefined", whereas the identity check foo === undefined would trigger the error "foo is not defined".

https://stackoverflow.com/a/4726198/4317384

For example, the following is used in IE for parsing XML:

var x = new ActiveXObject("Microsoft.XMLDOM");

To check whether it has a loadXML method safely:

typeof x.loadXML === "undefined"; // Returns false

On the other hand:

x.loadXML === undefined; // Throws an error

UPDATE

Another advantage of the typeof check that I forgot to mention was that it also works with undeclared variables, which the foo === undefined check does not, and in fact throws a ReferenceError. Thanks to @LinusKleen for reminding me. For example:

typeof someUndeclaredVariable; // "undefined"
someUndeclaredVariable === undefined; // throws a ReferenceError

[ncoden]

Ok LGTM. Label it as a fix if there is a particular issue you can reproduce and resolve. As a style or refactor otherwise (I'm not sure what would be the more appropriate).