no-this-assign-in-render: Only fail if the assignment is to a reactive property

[bearfriend]

Alternative/addition to #213

• Walk the assignment tree looking for true this assignments only
• Compare first assignment member against declared reactive properties, failing only on match
• Add and modify tests for property comparisons

[bearfriend]

I don't work with a lot of this (including typescript) often, so suggestions and edits are welcome.

[bearfriend]

There doesn't seem to be a great way to get the computed value (i.e. x in this[x]) to compare against the properties, so we fail on this as well.

[bearfriend]

@43081j Can you have a look at this when you get a chance?

[bearfriend]

@43081j Any interest in moving this along?

[43081j]

Sorry I've been quite busy traveling recently. I'll try get to this in the next few days 👍

[43081j]

the render method of a lit element is really meant to be a 'pure' function, in that it will produce the same result with the same props/state every time

this does mean you're technically correct that we care most about reactive properties. but, really, I think we shouldn't be mutating this at all during render and should use the lifecycle callbacks instead

@justinfagnani do you have thoughts on this?

I think we should just fix the existing behaviour instead of constraining it to reactive properties.

Justin can confirm but I feel like render just shouldn't mutate at all, really (of this at least)

as far as I understand, you need this because we're incorrectly flagging things like foo(this).bar = 123 as a mutation of this. that is a bug and we should fix it for sure