createFragment warning when rendering a Date

[glenjamin]

<p>{new Date()}</p>

Looks to be hitting the typeof object check.

Not sure if it should cast to string or toISOString or just error.

[zpao]

Yes, this is the case where you're rendering an Object with keys, which is what we are deprecating. You should just stringify that yourself.

In this case you don't want to actually use createFragment so I could see the warning being a bit misleading. But we can't do typechecks to special case Date (could be your own types, etc). Maybe we should add something like "or convert to a primitive" to the warning.

[glenjamin]

One of our engineers was stuck on this for ages, as the error examples all show very different scenarios.

Perhaps include obj.toString() in the warning if it isn't [object Object] or some similar heuristic?

Alternatively, what if the warning was for instanceof Object and everything else got coerced to a string?

[sophiebits]

We can improve this error a little bit.

[zpao]

I doubt we're going to start coercing here (eg, why wouldn't {toString: function() { return 'foo' } } be allowed) and the instanceof check isn't good enough (Object.create(null) instanceof Object === false).

Another helpful step with debugging: turn on pausing on caught exceptions. We throw and catch an error when warning to help with specifically this case.

Having more information in the warning is probably the best thing.

[glenjamin]

This actually popped up as we were adding unit tests to a component so we had a full stack trace (we convert warnings to thrown exceptions)- one of the downsides of the separate declare/render phases is that the stack trace rarely points to the relevant line in render() with errors like this.

We figured things out by doing as you suggest - inspecting exactly what value had caused React to decide to trigger the warning.

I think this suggests that including some representation of the erroring value might be the best addition to the warning?