Throw a better error when Lazy/Promise is used in React.Children #28280
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| case REACT_LAZY_TYPE: | ||
| throw new Error( | ||
| 'Cannot render an Async Component, Promise or React.Lazy inside React.Children. ' + | ||
| 'We recommend not iterating over children and just rendering them plain.', | ||
| ); |
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This makes it an error to iterate over Children if any child happens to be one of these things but you may not know that until you iterate over it. Basically it makes using Children risky if you don't control the inputs very carefully. The language here says you can't render but forEach allows you to do arbitrary stuff with the child so perhaps you will omit any non-renderable children.
This feels halfway towards deprecating React.Children but where you may learn about it in painful ways
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To be clear this is already going to error below so it's not a new change. It's just a better message.
The thing is that you can't omit the lazy because if you did something with an element you should've done it with the unwrapped lazy.
But yea, we've already effectively deprecated React.Children. We're just ambivalent about it.
gnoff
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) We could in theory actually support this case by throwing a Promise when it's used inside a render. Allowing it to be synchronously unwrapped. However, it's a bit sketchy because we officially only support this in the render's child position or in `use()`. Another alternative could be to actually pass the Promise/Lazy to the callback so that you can reason about it and just return it again or even unwrapping with `use()` - at least for the forEach case maybe. DiffTrain build for [e41ee9e](e41ee9e)
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Feb 9, 2024
Updates React from 2bc7d336a to ba5e6a832. ### React upstream changes - facebook/react#28283 - facebook/react#28280 - facebook/react#28079 - facebook/react#28233 - facebook/react#28276 - facebook/react#28272 - facebook/react#28265 - facebook/react#28259 - facebook/react#28153 - facebook/react#28246 - facebook/react#28218 - facebook/react#28263 - facebook/react#28257 - facebook/react#28261 - facebook/react#28262 - facebook/react#28260 - facebook/react#28258 - facebook/react#27864 - facebook/react#28254 - facebook/react#28219 - facebook/react#28248 - facebook/react#28216 - facebook/react#28249 - facebook/react#28241 - facebook/react#28243 - facebook/react#28253 - facebook/react#28256 - facebook/react#28236 - facebook/react#28237 - facebook/react#28242 - facebook/react#28251 - facebook/react#28252 Closes NEXT-2411
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This pains me because `React.Children` is really already pseudo-deprecated. `React.Children` takes any children that `React.Node` takes. We now support Lazy and Thenable in this position elsewhere, but it errors in `React.Children`. This becomes an issue with async Server Components which can resolve into a Lazy and in the future Lazy will just become Thenables. Which causes this to error. There are a few different semantics we could have: 1) Error like we already do (#28280). `React.Children` is about introspecting children. It was always sketchy because you can't introspect inside an abstraction anyway. With Server Components we fold away the components so you can actually introspect inside of them kind of but what they do is an implementation detail and you should be able to turn it into a Client Component at any point. The type of an Element passing the boundary actually reduces to `React.Node`. 2) Suspend and unwrap the Node (this PR). If we assume that Children is called inside of render, then throwing a Promise if it's not already loaded or unwrapping would treat it as if it wasn't there. Just like if you rendered it in React. This lets you introspect what's inside which isn't really something you should be able to do. This isn't compatible with deprecating throwing-a-Promise and enable static compilation like `use()` does. We'd have to deprecate `React.Children` before doing that which we might anyway. 3) Wrap in a Fragment. If a Server Component was instead a Client Component, you couldn't introspect through it anyway. Another alternative might be to let it pass through but then it wouldn't be given a flat key. We could also wrap it in a Fragment that is keyed. That way you're always seeing an element. The issue with this solution is that it wouldn't see the key of the Server Component since that gets forwarded to the child that is yet to resolve. The nice thing about that strategy is it doesn't depend on throw-a-Promise but it might not be keyed correctly when things move.
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This pains me because `React.Children` is really already pseudo-deprecated. `React.Children` takes any children that `React.Node` takes. We now support Lazy and Thenable in this position elsewhere, but it errors in `React.Children`. This becomes an issue with async Server Components which can resolve into a Lazy and in the future Lazy will just become Thenables. Which causes this to error. There are a few different semantics we could have: 1) Error like we already do (#28280). `React.Children` is about introspecting children. It was always sketchy because you can't introspect inside an abstraction anyway. With Server Components we fold away the components so you can actually introspect inside of them kind of but what they do is an implementation detail and you should be able to turn it into a Client Component at any point. The type of an Element passing the boundary actually reduces to `React.Node`. 2) Suspend and unwrap the Node (this PR). If we assume that Children is called inside of render, then throwing a Promise if it's not already loaded or unwrapping would treat it as if it wasn't there. Just like if you rendered it in React. This lets you introspect what's inside which isn't really something you should be able to do. This isn't compatible with deprecating throwing-a-Promise and enable static compilation like `use()` does. We'd have to deprecate `React.Children` before doing that which we might anyway. 3) Wrap in a Fragment. If a Server Component was instead a Client Component, you couldn't introspect through it anyway. Another alternative might be to let it pass through but then it wouldn't be given a flat key. We could also wrap it in a Fragment that is keyed. That way you're always seeing an element. The issue with this solution is that it wouldn't see the key of the Server Component since that gets forwarded to the child that is yet to resolve. The nice thing about that strategy is it doesn't depend on throw-a-Promise but it might not be keyed correctly when things move. DiffTrain build for [9e7944f](9e7944f)
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…ebook#28280) We could in theory actually support this case by throwing a Promise when it's used inside a render. Allowing it to be synchronously unwrapped. However, it's a bit sketchy because we officially only support this in the render's child position or in `use()`. Another alternative could be to actually pass the Promise/Lazy to the callback so that you can reason about it and just return it again or even unwrapping with `use()` - at least for the forEach case maybe.
EdisonVan
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…book#28284) This pains me because `React.Children` is really already pseudo-deprecated. `React.Children` takes any children that `React.Node` takes. We now support Lazy and Thenable in this position elsewhere, but it errors in `React.Children`. This becomes an issue with async Server Components which can resolve into a Lazy and in the future Lazy will just become Thenables. Which causes this to error. There are a few different semantics we could have: 1) Error like we already do (facebook#28280). `React.Children` is about introspecting children. It was always sketchy because you can't introspect inside an abstraction anyway. With Server Components we fold away the components so you can actually introspect inside of them kind of but what they do is an implementation detail and you should be able to turn it into a Client Component at any point. The type of an Element passing the boundary actually reduces to `React.Node`. 2) Suspend and unwrap the Node (this PR). If we assume that Children is called inside of render, then throwing a Promise if it's not already loaded or unwrapping would treat it as if it wasn't there. Just like if you rendered it in React. This lets you introspect what's inside which isn't really something you should be able to do. This isn't compatible with deprecating throwing-a-Promise and enable static compilation like `use()` does. We'd have to deprecate `React.Children` before doing that which we might anyway. 3) Wrap in a Fragment. If a Server Component was instead a Client Component, you couldn't introspect through it anyway. Another alternative might be to let it pass through but then it wouldn't be given a flat key. We could also wrap it in a Fragment that is keyed. That way you're always seeing an element. The issue with this solution is that it wouldn't see the key of the Server Component since that gets forwarded to the child that is yet to resolve. The nice thing about that strategy is it doesn't depend on throw-a-Promise but it might not be keyed correctly when things move.
bigfootjon
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Apr 18, 2024
) We could in theory actually support this case by throwing a Promise when it's used inside a render. Allowing it to be synchronously unwrapped. However, it's a bit sketchy because we officially only support this in the render's child position or in `use()`. Another alternative could be to actually pass the Promise/Lazy to the callback so that you can reason about it and just return it again or even unwrapping with `use()` - at least for the forEach case maybe. DiffTrain build for commit e41ee9e.
bigfootjon
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Apr 18, 2024
This pains me because `React.Children` is really already pseudo-deprecated. `React.Children` takes any children that `React.Node` takes. We now support Lazy and Thenable in this position elsewhere, but it errors in `React.Children`. This becomes an issue with async Server Components which can resolve into a Lazy and in the future Lazy will just become Thenables. Which causes this to error. There are a few different semantics we could have: 1) Error like we already do (#28280). `React.Children` is about introspecting children. It was always sketchy because you can't introspect inside an abstraction anyway. With Server Components we fold away the components so you can actually introspect inside of them kind of but what they do is an implementation detail and you should be able to turn it into a Client Component at any point. The type of an Element passing the boundary actually reduces to `React.Node`. 2) Suspend and unwrap the Node (this PR). If we assume that Children is called inside of render, then throwing a Promise if it's not already loaded or unwrapping would treat it as if it wasn't there. Just like if you rendered it in React. This lets you introspect what's inside which isn't really something you should be able to do. This isn't compatible with deprecating throwing-a-Promise and enable static compilation like `use()` does. We'd have to deprecate `React.Children` before doing that which we might anyway. 3) Wrap in a Fragment. If a Server Component was instead a Client Component, you couldn't introspect through it anyway. Another alternative might be to let it pass through but then it wouldn't be given a flat key. We could also wrap it in a Fragment that is keyed. That way you're always seeing an element. The issue with this solution is that it wouldn't see the key of the Server Component since that gets forwarded to the child that is yet to resolve. The nice thing about that strategy is it doesn't depend on throw-a-Promise but it might not be keyed correctly when things move. DiffTrain build for commit 9e7944f.
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We could in theory actually support this case by throwing a Promise when it's used inside a render. Allowing it to be synchronously unwrapped. However, it's a bit sketchy because we officially only support this in the render's child position or in
use().Another alternative could be to actually pass the Promise/Lazy to the callback so that you can reason about it and just return it again or even unwrapping with
use()- at least for the forEach case maybe.