this.mirages is an array.

You are right but it's not easier and usual to use length form to iterate in an array?

I think it would be better to revert to the original for style like Polakrity suggests. for...in is better suited for dictionaries, and adding a + everywhere player is used in that function sounds like a pain and people might make the mistake again.

Should we discourage for...in on plain arrays in the coding conventions in general? It sounds saner to use it on dictionary-like objects only.

I guess we only need +player on 202 that's where the warning comes from.

In rP22247#33237, @Stan wrote:

I guess we only need +player on 202 that's where the warning comes from.

It's not a good idea to base oneself on the warnings, which are not always the root cause. Here indeed only this line will break because this is a call to a C++ component (so calling GetLosVisibility(..., "1") will fail), but on the other lines we are still calling stuff like this.mirages["1"], which, even if it is allowed in JS, is bad. player should just not be a string, which means we shouldn't use for in here.

I mean, in JS every key of an object is a string.

Though:

Should we discourage for...in on plain arrays in the coding conventions in general? It sounds saner to use it on dictionary-like objects only.

Sounds fair enough to me. The behaviour is a tad counterintuitive.
TBH I'm not sure where our up-to-date coding conventions are though.