Say something about undefined

I find people treat `undefined` incorrectly a lot. It's a value, you can use it, test for it, etc, but people frequently use `typeof x == "undefined"` when they don't need to.

I made a [fork](https://github.com/ianb/javascript) with a big changeset of my own opinions, but here's the section I wrote about undefined:

---

``````
## undefined

- `undefined` is an object. You should use it as an object. You can test for it. For instance:

```javascript
// good
function pow(a, b) {
if (b === undefined) {
b = 1;
}
return Math.pow(a, b);
}

// bad
function pow(a, b) {
if (typeof b == "undefined") {
b = 1;
}
return Math.pow(a, b);
}
```

- *Only* use `typeof x == "undefined"` when the variable (`x`) may not be declared, and it would be an error to test `x === undefined`:

```javascript
if (typeof Module == "undefined") {
Module = {};
}

// But also okay, for browser-only code:
if (window.Module === undefined) {
Module = {};
}
```

Note that you can't use `window` in Node.js; if you think your code could be used in a server context you should use the first form.
``````

Review the issue text and the linked fork's proposed undefined section first; no repository file or test is named. Confirm the project's preferred guidance for checking undefined, then update the relevant style-guide documentation if maintainers agree and verify that the final wording is consistent with the surrounding guidance.

Written by the indexing model from the issue text.