JS: Ignore JSON-like files in js/useless-expression

[erik-krogh]

The anomalous query results found a project with a lot of useless-expressions.

However, most of those results were from JSON-like files. And while the expressions are useless, the warning looks benign.

I added a condition on the js/useless-expression so it doesn't report an expression if the containing file only contains that expression.
That pattern is somewhat common.

I could possibly restrict it further, such that we still report a useless expression if the expression contains a function?

[esbena]

If the goal is to exclude json-like constructs only, then I think we should try a little harder to weed out illegal json. Some ideas: can we remove variable references, function literals, regexp literals, calls?

(We have previously tried to classify entire json-like files as generated, but we never found a scalable way to to that reliably.)

[asgerf]

Looking great! 👍

We should probably exclude CodeInAttribute top-levels from this check, otherwise LGTM.

[erik-krogh]

Some ideas: can we remove variable references, function literals, regexp literals, calls?

I've removed function literals from the check for now. When I look at the results we no longer flag I only want to remove functions from those.
The rest with e.g. binary expressions look like a benign parser-tests or something.

We should probably exclude CodeInAttribute top-levels from this check, otherwise LGTM.

I don't think there is any need for that.
Code in an attribute is already not in a void-context, and thus something like <div onclick="[1,2,3]"></div> is already not flagged.

[erik-krogh]

An evaluation looks fine.
Lots of parser tests are no longer flagged as useless.