handle the case where self._interval is undefined

[lacker]

In some situations, Node returns undefined for setInterval. This just adds a check to make sure we don't access properties on undefined

[lacker]

This PR fixes the problem that I reported in #481

[t-mullen]

I'm not against merging this if JSDom isn't being fixed. Seems like a potentially breaking change on their side.

[lacker]

Yeah, I don't know how long it will take to change JSDom - this is probably a low priority fix on their end. And it seems pretty harmless to fix it here. So IMO it would be clean just to merge this fix.

[nazar-pc]

I'd actually be against merging hacks like this, especially since it is possible to hack around it without changing simple-peer.

[lacker]

For what it's worth, it's not that JSDom itself is so popular that simple-peer should support it per se, it's that when I was using simple-peer in conjunction with Jest unit testing, the tests were crashing frequently outside of my code, and I tracked it down to this problem. So fixing this would let simple-peer work together with Jest unit tests. That seems popular enough that it is worth compatibility IMO.

[t-mullen]

Monkey patching window.setInterval outside of simple-peer is the solution until JSDom fixes this. You should do this anyways since this behaviour may break other libraries.

I know it seems easier to include this hack in simple-peer, but realize that every single JS library will need to do this as well.

jsdom/jsdom#2617

From spec:

The setInterval() method must return the value returned by the timer initialization steps ...a user-agent-defined integer that is greater than zero that will identify the timeout to be set by this call in the list of active timers.