Add historical note to no-typeof-undefined

[fregante]

This rule seemingly goes against the "best practice" of always using typeof. I'd like to explain that the practice is no longer "best", but I'm not sure if this is the best way to phrase it.

[fisker]

I don't know what that mean. I don't think use typeof or not related to the strict mode.

[fregante]

Before strict mode, undefined could be defined and undefined === 1 was entirely possible. For this reason, direct comparison with undefined was frowned upon and is probably still instinctively seen as a bad pattern today.

[fisker]

In strict mode const undefined = 1 still valid, only globalThis.undefined is readonly. Correct me if I'm wrong.

[fisker]

I believe the global undefined is readonly because it changed in ES5 https://262.ecma-international.org/5.1/#sec-15.1.1.3 spec.

It has nothing to do with the strict mode.

$ node -p "(function() {'use strict';const undefined = 1; return typeof undefined})()"
number

$ node -p "(function() {const undefined = 1; return typeof undefined})()"
number

[fregante]

Gotcha, so the rule was related to pre-ES5 browsers. I'll update the wording

[fregante]

Is this more correct? It seems that IE9 did this correctly already according to http://kangax.github.io/compat-table/es5/ (IE8 tests missing)

[fregante]

Indeed broken on IE8 👍

[fregante]

Brokener on IE9 😂

[fisker]

Not sure about this, I use typeof(since long time ago, maybe before IE6) not because I'm worried about undefined reassigned, but worried about error thrown on undefined global variable. But I guess it's fine to add this to the doc. Won't block.