ReactIs.typeOf for non-elements

[mridgway]

What is the current behavior?

ReactIs.typeOf currently only works for element types, it does not allow you to pass a raw Component or ForwardRef to know the type.

The use case for this is in hoist-non-react-statics I now need a special cases for ForwardRefs. To do this, I will need to know the type of the source and target components, but currently I would need to turn them into elements first.

All of the ReactIs.is* functions also have this issue since they use the typeOf function internally.

const ForwardComponent = React.forwardRef(() => {});

ReactIs.typeOf(ForwardComponent); // undefined
ReactIs.typeOf(React.createElement(ForwardComponent)); // Symbol(react.forward_ref)

What is the expected behavior?

Ideally I could pass in just the Component and get the type of it:

ReactIs.typeOf(ForwardComponent) // Symbol(react.forward_ref)

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?

ReactIs@16.3.2

[aweary]

Hey @mridgway, thanks for bringing this up. As I understand it, the intention with ReactIs.typeOf is that it reads the types of different elements; it's meant to type check values that are returned from React.createElement. There's also ReactIs.isValidElementType which tells you whether a value can be passed to React.createElement. So there's a certain symmetry there.

Changing typeOf to return the type for the ForwardRef element type and ForwardRef elements could create problems if anyone relies on typeOf to only return values for things that can be rendered (things returned from React.createElement). Maybe that's OK in this case, I'm not sure.

It sounds like what you want is a hybrid of the two that tells you the type for element types.

React.elementType('div')
// Symbol(react.element)
React.elementType(React.StrictMode)
// Symbol(react.strict_mode)
React.elementType(React.forwardRef(Component))
// Symbol(react.forward_ref)

Do you think an API like that would suffice for your use case(s)? @bvaughn what do you think?

[bvaughn]

As I understand it, the intention with ReactIs.typeOf is that it reads the types of different elements; it's meant to type check values that are returned from React.createElement.

That's correct.

I can see the usefulness of a method like the one @aweary describes. I don't really have strong thoughts about it other than sharing Brandon's concern about it being a backwards breaking change to make to the existing react-is methods.

[mridgway]

@aweary Yes, I think that API would be sufficient for my use case. Specifically I would be doing something like if (React.elementType(React.forwardRef(Component)) === ReactIs.ForwardRef). Any way that I can do this without having to reach in to Component['$$typeof'] would be fine with me.

[bvaughn]

Any way that I can do this without having to reach in to Component['$$typeof'] would be fine with me.

ReactIs.isForwardRef(
  React.createElement(
    YourComponent
  )
)

🤡

[mridgway]

Yeah, that's what I will do for now. It seems a bit roundabout though and also introduces a dependency on React that I wouldn't need if ReactIs contained all of the logic.

Edit: Just realized that was a clown icon. It did not show up on my Mac 😞

[aweary]

I took a stab at implementing elementType #12932

[mridgway]

Checking in on this again. Since it does not seem like there is movement on adding an API to be able to check types without creating elements, I will be implementing my changes using React.createElement.

This brings up the question of what the compatibility for react-is is currently and what its plans are in the future. If I need to add react-is and react as dependencies, can I loosely version react but tightly version react-is or will react-is continue to version in lock-step with react? Which previous versions of react does react-is support?

[bvaughn]

Which previous versions of react does react-is support?

Should support versions 0.14+

It relies on $$typeof (added in 0.14), so it will not support <= 0.13

[mridgway]

Looks like using React.createElement is also problematic since it creates warnings if you don't have the correct props added.

[mweststrate]

For future inspiration / real-life use case / other running into this as well:

In the mobx-react bindings some special forward ref handling was required as well, but react-is turned out to be not really valuable as it works on elements, not on types. Since the ForwardRef symbol isn't officially exposed from the package (or at least it become undefined after using rollup, didn't investigate further), in the end it was solved like:

https://github.com/mobxjs/mobx-react/blob/a3e9d3928015511bcb14846aad4de21a219ed39a/src/observer.js#L30-L31

const ReactForwardRefSymbol =
    typeof forwardRef === "function" && forwardRef((_props, _ref) => {})["$$typeof"]

and:

https://github.com/mobxjs/mobx-react/blob/a3e9d3928015511bcb14846aad4de21a219ed39a/src/observer.js#L329

if (ReactForwardRefSymbol && componentClass["$$typeof"] === ReactForwardRefSymbol) {
  // something specific
}

Doesn't feel entirely kosher, especially the dependency on $$typeof, so a suggestion for better alternatives would be appreciated but this does seem to do the trick for now.

(N.B.: there is an interesting underlying reason for the special case handling of refs, the bindings are "unpacking" the forward ref to make sure observer is applied as first HoC, normally that responsibility is moved to user land; but with forward refs that can't be really done (ForwardRefs are often produced by other libs and the user is unaware of them). If this problem is worth discussing I could open another issue)