RFC: Plan for custom element attributes/properties in React 19

[robdodson]

This is meant to address #7249. The doc outlines the pros and cons of various approaches React could use to handle attributes and properties on custom elements.

TOC/Summary

• Background
• Proposals
  • Option 1: Only set properties
    • Pros
      • Easy to understand/implement
      • Avoids conflict with future global attributes
      • Takes advantage of custom element "upgrade"
      • Custom elements treated like any other React component
    • Cons
      • Possibly a breaking change
      • Need ref to set attribute
      • Not clear how server-side rendering would work
  • Option 2: Properties-if-available
    • Pros
      • Non-breaking change
    • Cons
      • Developers need to understand the heuristic
      • Falling back to attributes may conflict with future globals
  • Option 3: Differentiate properties with a sigil
    • Pros
      • Non-breaking change that developers can opt-in to
      • Similar to how other libraries handle attributes/properties
      • The system is explicit
    • Cons
      • It’s new syntax
      • Not clear how server-side rendering would work
  • Option 4: Add an attributes object
    • Pros
      • The system is explicit
      • Extending syntax may also solve issues with event handling
    • Cons
      • It’s new syntax
      • It may be a breaking change
      • It may be a larger change than any of the previous proposals
  • Option 5: An API for consuming custom elements
    • Pros
      • The system is explicit
      • Non-breaking change
      • Idiomatic to React
    • Cons
      • Could be a lot of work for a complex component
      • May bloat bundle size
      • Config needs to keep pace with the component

Background

When React tries to pass data to a custom element it always does so using HTML attributes.

<x-foo bar={baz}> // same as setAttribute('bar', baz)

Because attributes must be serialized to strings, this approach creates problems when the data being passed is an object or array. In that scenario, we end up with something like:

<x-foo bar="[object Object]">

The workaround for this is to use a ref to manually set the property.

<x-foo ref={el => el.bar = baz}>

This workaround feels a bit unnecessary as the majority of custom elements being shipped today are written with libraries which automatically generate JavaScript properties that back all of their exposed attributes. And anyone hand-authoring a vanilla custom element is encouraged to follow this practice as well. We'd like to ideally see runtime communication with custom elements in React use JavaScript properties by default.

This doc outlines a few proposals for how React could be updated to make this happen.

Proposals

Option 1: Only set properties

Rather than try to decide if a property or attribute should be set, React could always set properties on custom elements. React would NOT check to see if the property exists on the element beforehand.

Example:

<x-foo bar={baz}>

The above code would result in React setting the .bar property of the x-foo element equal to the value of baz.

For camelCased property names, React could use the same style it uses today for properties like tabIndex.

<x-foo squidInk={pasta}> // sets .squidInk = pasta

Pros

Easy to understand/implement

This model is simple, explicit, and dovetails with React’s "JavaScript-centric API to the DOM".

Any element created with libraries like Polymer or Skate will automatically generate properties to back their exposed attributes. These elements should all "just work" with the above approach. Developers hand-authoring vanilla components are encouraged to back attributes with properties as that mirrors how modern (i.e. not oddballs like <input>) HTML5 elements (<video>, <audio>, etc.) have been implemented.

Avoids conflict with future global attributes

When React sets an attribute on a custom element there’s always the risk that a future version of HTML will ship a similarly named attribute and break things. This concern was discussed with spec authors but there is no clear solution to the problem. Avoiding attributes entirely (except when a developer explicitly sets one using ref) may sidestep this issue until the browsers come up with a better solution.

Takes advantage of custom element "upgrade"

Custom elements can be lazily upgraded on the page and some PRPL patterns rely on this technique. During the upgrade process, a custom element can access the properties passed to it by React—even if those properties were set before the definition loaded—and use them to render initial state.

Custom elements treated like any other React component

When React components pass data to one another they already use properties. This would just make custom elements behave the same way.

Cons

Possibly a breaking change

If a developer has been hand-authoring vanilla custom elements which only have an attributes API, then they will need to update their code or their app will break. The fix would be to use a ref to set the attribute (explained below).

Need ref to set attribute

By changing the behavior so properties are preferred, it means developers will need to use a ref in order to explicitly set an attribute on a custom element.

<custom-element ref={el => el.setAttribute('my-attr', val)} />

This is just a reversal of the current behavior where developers need a ref in order to set a property. Since developers should rarely need to set attributes on custom elements, this seems like a reasonable trade-off.

Not clear how server-side rendering would work

It's not clear how this model would map to server-side rendering custom elements. React could assume that the properties map to similarly named attributes and attempt to set those on the server, but this is far from bulletproof and would possibly require a heuristic for things like camelCased properties -> dash-cased attributes.

Option 2: Properties-if-available

At runtime React could attempt to detect if a property is present on a custom element. If the property is present React will use it, otherwise it will fallback to setting an attribute. This is the model Preact uses to deal with custom elements.

Pseudocode implementation:

if (propName in element) {
  element[propName] = value;
} else {
  element.setAttribute(propName.toLowerCase(), value);
}

Possible steps:

• If an element has a defined property, React will use it.

• If an element has an undefined property, and React is trying to pass it primitive data (string/number/boolean), it will use an attribute.

  • Alternative: Warn and don’t set.

• If an element has an undefined property, and React is trying to pass it an object/array it will set it as a property. This is because some-attr="[object Object]” is not useful.

  • Alternative: Warn and don’t set.

• If the element is being rendered on the server, and React is trying to pass it a string/number/boolean, it will use an attribute.

• If the element is being rendered on the server, and React is trying to pass it a object/array, it will not do anything.

Pros

Non-breaking change

It is possible to create a custom element that only uses attributes as its interface. This authoring style is NOT encouraged, but it may happen regardless. If a custom element author is relying on this behavior then this change would be non-breaking for them.

Cons

Developers need to understand the heuristic

Developers might be confused when React sets an attribute instead of a property depending on how they’ve chosen to load their element.

Falling back to attributes may conflict with future globals

Sebastian raised a concern that using in to check for the existence of a property on a custom element might accidentally detect a property on the superclass (HTMLElement).

There are also other potential conflicts with global attributes discussed previously in this doc.

Option 3: Differentiate properties with a sigil

React could continue setting attributes on custom elements, but provide a sigil that developers could use to explicitly set properties instead. This is similar to the approach used by Glimmer.js.

Glimmer example:

<custom-img @src="corgi.jpg" @hiResSrc="corgi@2x.jpg" width="100%">

In the above example, the @ sigil indicates that src and hiResSrc should pass data to the custom element using properties, and width should be serialized to an attribute string.

Because React components already pass data to one another using properties, there would be no need for them to use the sigil (although it would work if they did, it would just be redundant). Instead, it would primarily be used as an explicit instruction to pass data to a custom element using JavaScript properties.

h/t to @developit of Preact for suggesting this approach :)

Pros

Non-breaking change that developers can opt-in to

All pre-existing React + custom element apps would continue to work exactly as they have. Developers could choose if they wanted to update their code to use the new sigil style.

Similar to how other libraries handle attributes/properties

Similar to Glimmer, both Angular and Vue use modifiers to differentiate between attributes and properties.

Vue example:

<!-- Vue will serialize `foo` to an attribute string, and set `squid` using a JavaScript property -->
<custom-element :foo="bar” :squid.prop=”ink”>

Angular example:

<!-- Angular will serialize `foo` to an attribute string, and set `squid` using a JavaScript property -->
<custom-element [attr.foo]="bar” [squid]=”ink”>

The system is explicit

Developers can tell React exactly what they want instead of relying on a heuristic like the properties-if-available approach.

Cons

It’s new syntax

Developers need to be taught how to use it and it needs to be thoroughly tested to make sure it is backwards compatible.

Not clear how server-side rendering would work

Should the sigil switch to using a similarly named attribute?

Option 4: Add an attributes object

React could add additional syntax which lets authors explicitly pass data as attributes. If developers do not use this attributes object, then their data will be passed using JavaScript properties.

Example:

const bar = 'baz';
const hello = 'World';
const width = '100%';
const ReactElement = <Test
  foo={bar} // uses JavaScript property
  attrs={{ hello, width }} // serialized to attributes
/>;

This idea was originally proposed by @treshugart, author of Skate.js, and is implemented in the val library.

Pros

The system is explicit

Developers can tell React exactly what they want instead of relying on a heuristic like the properties-if-available approach.

Extending syntax may also solve issues with event handling

Note: This is outside the scope of this document but maybe worth mentioning :)

Issue #7901 requests that React bypass its synthetic event system when declarative event handlers are added to custom elements. Because custom element event names are arbitrary strings, it means they can be capitalized in any fashion. To bypass the synthetic event system today will also mean needing to come up with a heuristic for mapping event names from JSX to addEventListener.

// should this listen for: 'foobar', 'FooBar', or 'fooBar'?
onFooBar={handleFooBar}

However, if the syntax is extended to allow attributes it could also be extended to allow events as well:

const bar = 'baz';
const hello = 'World';
const SquidChanged = e => console.log('yo');
const ReactElement = <Test
  foo={bar}
  attrs={{ hello }}
  events={{ SquidChanged}} // addEventListener('SquidChanged', …)
/>;

In this model the variable name is used as the event name. No heuristic is needed.

Cons

It’s new syntax

Developers need to be taught how to use it and it needs to be thoroughly tested to make sure it is backwards compatible.

It may be a breaking change

If any components already rely on properties named attrs or events, it could break them.

It may be a larger change than any of the previous proposals

For React 17 it may be easier to make an incremental change (like one of the previous proposals) and position this proposal as something to take under consideration for a later, bigger refactor.

Option 5: An API for consuming custom elements

This proposal was offered by @sophiebits and @gaearon from the React team

React could create a new API for consuming custom elements that maps the element’s behavior with a configuration object.

Pseudocode example:

const XFoo = ReactDOM.createCustomElementType({
  element: ‘x-foo’,
  ‘my-attr’: // something that tells React what to do with it
  someRichDataProp: // something that tells React what to do with it
});

The above code returns a proxy component, XFoo that knows how to pass data to a custom element depending on the configuration you provide. You would use this proxy component in your app instead of using the custom element directly.

Example usage:

<XFoo someRichDataProp={...} />

Pros

The system is explicit

Developers can tell React the exact behavior they want.

Non-breaking change

Developers can opt-in to using the object or continue using the current system.

Idiomatic to React

This change doesn’t require new JSX syntax, and feels more like other APIs in React. For example, PropTypes (even though it’s being moved into its own package) has a somewhat similar approach.

Cons

Could be a lot of work for a complex component

Polymer’s paper-input element has 37 properties, so it would produce a very large config. If developers are using a lot of custom elements in their app, that may equal a lot of configs they need to write.

May bloat bundle size

Related to the above point, each custom element class now incurs the cost of its definition + its config object size.

Note: I'm not 100% sure if this is true. Someone more familiar with the React build process could verify.

Config needs to keep pace with the component

Every time the component does a minor version revision that adds a new property, the config will need to be updated as well. That’s not difficult, but it does add maintenance. Maybe if configs are generated from source this is less of a burden, but that may mean needing to create a new tool to generate configs for each web component library.

cc @sebmarkbage @gaearon @developit @treshugart @justinfagnani

[robdodson]

Apologies for the long read, but I wanted to make sure I was thoroughly exploring each option. I don't want to bias things too much with my own opinion, but if I were in a position to choose, I think I'd go with option 3.

Option 3 is backwards compatible, declarative, and explicit. There’s no need to maintain a fallback heuristic, and other libraries already provide similar sigils/modifiers.

[worawit15379]

Apologies for the long read, but I wanted to make sure I was thoroughly exploring each option. I don't want to bias things too much with my own opinion, but if I were in a position to choose, I think I'd go with option 3. Option 3 is backwards compatible, declarative, and explicit. There’s no need to maintain a fallback heuristic, and other libraries already provide similar sigils/modifiers.

[jeremenichelli]

I'm between option 2 and option 3, I think that React has handled behavior and API changes very well in the past. Introducting warnings and links to docs might serve well to help developers understand what's happening under the hood.

Option 3 looks attractive because of its declarative nature, while reading JSX code new coming developers will know immediately what React will do when rendering the element.

[cjorasch]

Comments on option 2

Developers might be confused when React sets an attribute instead of a property depending on how they’ve chosen to load their element.

Do consumers of a custom element need to understand this distinction? Or is that only important to the author of the custom element? It seems like the author of the element will need to handle attributes for anything used in HTML (since that is the only way data gets passed from HTML usage) and properties if they want to support complex values or property get/set from DOM. It is even possible an author could have something initially implemented as an attribute and then later add a property with the same name to support more flexible data types and still back the property with a value stored in the attributes.

Naming collisions with future HTMLElement attributes and properties seems like a weakness in the Web Components standards in general since that can lead to errors regardless of the binding approach.

If an element has an undefined property, and React is trying to pass it an object/array it will set it as a property. This is because some-attr="[object Object]” is not useful.

It seems confusing to bind differently based on the value. If the author of the element has not specified a property getter/setter to handle the value then setting the property would cause the element to behave like the value was never specified which might be harder to debug.

Comments on option 3

Another potential con with option 3 is that it requires the consumer of the custom element to know whether the element has implemented something as a property or as an attribute. If you are using a mix of React components and custom elements it could be confusing to set React props using one syntax and custom element properties using a different syntax.

[robdodson]

Do consumers of a custom element need to understand this distinction? Or is that only important to the author of the custom element?

I doubt it's actually a huge issue because, as you pointed out, the element author should define an attribute and property for the underlying value and accept data from both. I would also add that they should keep the attribute and property in sync (so setting one sets the other).

Naming collisions with future HTMLElement attributes and properties seems like a weakness in the Web Components standards in general since that can lead to errors regardless of the binding approach.

I agree but I'm not sure if this is something React needs to try to work around in their library. It feels like a problem that needs to be solved as part of the custom elements spec. I can see if we can discuss it as part of the upcoming TPAC standards meeting.

I should add, for properties this isn't as bad because the element-defined property will shadow the future property added to HTMLElement. So if you were passing data to a custom element as a js property, it would continue to work. The main issue seems to be around attributes since they are global.

It seems confusing to bind differently based on the value. If the author of the element has not specified a property getter/setter to handle the value then setting the property would cause the element to behave like the value was never specified which might be harder to debug.

In the case where a custom element is lazy loaded and "upgraded", it will initially have undefined properties. This addresses that use case by making sure those elements still receive their data and they can use it post-upgrade.

It's true that if the author doesn't define a getter/setter for a value this would not be very useful. But it's also not useful to have an my-attr=[object Object]. And since you don't know if the property is truly undefined or if they definition is just being lazy loaded, it seems safest to set the property.

Another potential con with option 3 is that it requires the consumer of the custom element to know whether the element has implemented something as a property or as an attribute.

I think you're essentially in the same boat today because there's nothing that forces a custom element author to define an attribute instead of a property. So I could have an element with a properties only API that would not receive any data from React's current system and I would need to know to use ref to directly set the js properties.

Because custom elements are meant as a primitive, there's nothing that enforces creating corresponding attributes and properties. But we're trying very hard to encourage doing so as a best practice, and all of the libraries that I know of today create backing properties for their attributes.

[edit]

As you stated in your earlier point:

It seems like the author of the element will need to handle attributes for anything used in HTML (since that is the only way data gets passed from HTML usage) and properties if they want to support complex values or property get/set from DOM.

Because you never know how a user will try to pass data to your element, you end up needing to have attribute-property correspondence anyway. I imagine if option 3 shipped that most folks would just bind everything using the @ sigil because it'd be easiest. That's how I work with custom elements in Vue today since they expose a .prop modifier.

[jeremenichelli]

it requires the consumer of the custom element to know whether the element has implemented something as a property or as an attribute

That's not something React should worry as Rob said in my opinion, it's the custom element author's responsability to inform the user how the element works.

And it's actually the way that we need to do it today, for example think about the <video> element, let's say you need to mute it or change the current time inside a component.

muted works as a boolean attribute

render() {
  return (
    <div className="video--wrapper">
      <video muted={ this.state.muted } />
    </div>
  );
}

For the current time you need to create a ref pointing to the video element and change the property.

render() {
  return (
    <div className="video--wrapper">
      <video ref={ el => this.video = el } muted={ this.state.muted } />
    </div>
  );
}

Then create an event handler, an instance method and manually set the property to the DOM element.

onCurrenTimeChange(e) {
  this.video.currentTime = e.value;
}

If you think about it it kinda breaks the declarative model React itself imposes with its API and JSX abstract layer since the currentTime it's clearly a state in the wrapper component, with property binding we would still need the event handler but the JSX abstraction model would be more declarative and refs wouldn't be necessary just for this:

render() {
  return (
    <div className="video--wrapper">
      <video muted={ this.state.muted } @currentTime={ this.state.currentTime } />
    </div>
  );
}

My point is that whether you are relying on native or custom elements, you still need to know your way around them based on documentation, the difference that in the second case it should come from the custom element's author.

@cjorasch my two cents :)

[effulgentsia]

If we were designing this from scratch, without needing to consider backwards compatibility, I think option 1 would be the most idiomatic per React’s "JavaScript-centric API to the DOM".

With regard to server-side rendering, could that problem be solved by providing an API for application code to inform React on how to map custom element properties to attributes? Similar to the maps that React already maintains for platform-defined attributes? This API would only need to be invoked once per custom element name (not for each instance of it), and only for properties that don't follow a straight 1:1 correspondence with their attribute, which should hopefully be relatively rare.

If we're concerned about this being too much of a breaking change though, then I think option 3 is pretty appealing as well. If the sigil signifies a property, I would suggest ".", since that's already JavaScript's property accessor. However, I think it's unfortunate to make every instance of where a custom element is used in an application be responsible for knowing when to use an attribute vs. when to use a property. What I prefer about option 1 is that even if a property to attribute map is needed, that mapping code can be isolated from all the JSX usages.

[cjorasch]

In the case where a custom element is lazy loaded and "upgraded", it will initially have undefined properties. This addresses that use case by making sure those elements still receive their data and they can use it post-upgrade.

Maybe I don't understand the upgrade process. Elements would typically have properties defined as getters/setters in the class prototype. Checking propName in element would return true because of the existence of the getter/setter even if the property value was still undefined. During upgrade do property values get set on some temporary instance and then later get copied to the actual instance once the lazy load is complete?

[effulgentsia]

Upgrading is the process by which the custom element receives its class. Prior to that, it's not an instance of that class, so the property getters/setters aren't available.

[robdodson]

@jeremenichelli

muted works as a boolean attribute

just checked and it also has a corresponding property though it doesn't seem to be documented on MDN :P

For the current time you need to create a ref pointing to the video element and change the property.

Yeah occasionally you'll encounter properties-only APIs on modern HTML elements. currentTime updates at a high frequency so it wouldn't make sense to reflect it to an HTML attribute.

My point is that wether you are relying on native or custom elements, you still need to know your way around them based on documentation

Yep there's unfortunately no one-size-fits-all attributes/properties rule. But I think generally speaking you can lean heavily on properties and provide syntax so developers can use attributes in special cases.

[jeremenichelli]

@robdodson yeap, I knew about the muted property too 😄 I just used these two to prove that already in the wild there isn't a one-size-fits-all rule as you mentioned.

We will have to rely on documentation on both native and custom elements, so it's something I wouldn't mind for this decision.

While writing the last code snippet I kinda liked the property binding though 💟

[robdodson]

@effulgentsia

However, I think it's unfortunate to make every instance of where a custom element is used in an application be responsible for knowing when to use an attribute vs. when to use a property.

I think this is already the case today though. Since the major custom element libraries (polymer, skate, possibly others?) automatically create backing properties for all exposed attributes, developers could just use the sigil for every property on a custom element. It would probably be a rare occurrence for them to need to switch to using an attribute.

@cjorasch

RE: upgrade. As @effulgentsia mentioned, it's possible to have a custom element on the page but load its definition at a later time. <x-foo> will initially be an instance of HTMLElement and when I load its definition later it "upgrades" and becomes an instance of the XFoo class. At this point all of its lifecycle callbacks get executed. We use this technique in the Polymer Starter Kit project. Kind of like this:

<app-router>
  <my-view1></my-view1>
  <my-view2></my-view2>
</app-router>

In the above example, we won't load the definition for my-view2 until the router changes to it.

It's entirely possible to set a property on the element before it has upgraded, and once the definition is loaded the element can grab that data during one of its lifecycle callbacks.

[effulgentsia]

developers could just use the sigil for every property on a custom element

If developers started doing that, then how would that differentiate using a property because you "can" from using a property because you "must"? And isn't that a differentiation that's needed for server-side rendering?

[robdodson]

If developers started doing that, then how would that differentiate using a property because you "can" from using a property because you "must"?

Sorry, maybe I phrased that wrong. I meant that developers would likely use the sigil because it would give the most consistent result. You can use it to pass primitive data or rich data like objects and arrays and it'll always work. I think working with properties at runtime is generally preferred to working with attributes since attributes tend to be used more for initial configuration.

And isn't that a differentiation that's needed for server-side rendering?

It might be the case that on the server the sigil would fallback to setting an attribute.

[effulgentsia]

It might be the case that on the server the sigil would fallback to setting an attribute.

I don't think that would work if the reason for the sigil is that it's a property that doesn't exist as an attribute, such as video's currentTime.

differentiate using a property because you "can" from using a property because you "must"

I think this differentiation is important, because there's entirely different reasons for choosing to use an attribute or property as an optimization (e.g., SSR preferring attributes vs. client-side rendering preferring properties) vs. something that exists either as only an attribute or only a property.

With regard to server-side rendering, could that problem be solved by providing an API for application code to inform React on how to map custom element properties to attributes?

To be more specific, I'm suggesting something like this:

ReactDOM.defineCustomElementProp(elementName, propName, domPropertyName, htmlAttributeName, attributeSerializer)

Examples:

// 'muted' can be set as either a property or an attribute.
ReactDOM.defineCustomElementProp('x-foo', 'muted', 'muted', 'muted')

// 'currentTime' can only be set as a property.
ReactDOM.defineCustomElementProp('x-foo', 'currentTime', 'currentTime', null)

// 'my-attribute' can only be set as an attribute.
ReactDOM.defineCustomElementProp('x-foo', 'my-attribute', null, 'my-attribute')

// 'richData' can be set as either a property or an attribute.
// When setting as an attribute, set it as a JSON string rather than "[object Object]".
ReactDOM.defineCustomElementProp('x-foo', 'richData', 'richData', 'richdata', JSON.stringify)

For something that can only be a property (where htmlAttributeName is null), SSR would skip over rendering it and then hydrate it on the client.

For something that can only be an attribute (where domPropertyName is null), React would invoke setAttribute() as currently in v16.

For something that can be both, React could choose whatever strategy is most optimal. Perhaps that means always setting as a property on client-side, but as an attribute server-side. Perhaps it means setting as an attribute when initially creating the element, but setting as a property when later patching from the vdom. Perhaps it means only setting as an attribute when the value is a primitive type. Ideally, React should be able to change the strategy whenever it wants to as an internal implementation detail.

When React encounters a prop for which defineCustomElementProp() hasn't been called and which isn't defined by the HTML spec as a global property or attribute, then React can implement some default logic. For example, perhaps:

• In version 17, maintain BC with v16 and set as an attribute.
• In version 18, assume that it can be either and follow the most optimal strategy for that.

But in any case, by keeping this a separate API, the JSX and props objects are kept clean and within a single namespace, just like they are for React components and non-custom HTML elements.

[effulgentsia]

Sorry for the excessive comments, but I thought of another benefit to my proposal above that I'd like to share:

Those ReactDOM.defineCustomElementProp() calls could be provided in a JS file maintained by the custom element author (in the same repository as where the custom element is maintained/distributed). It wouldn't be needed for custom elements with a strict 1:1 correspondence of property/attribute, which per this issue's Background statement is the recommendation and majority case anyway. So only custom element authors not following this recommendation would need to provide the React integration file. If the author doesn't provide it (e.g., because the custom element author doesn't care about React), then the community of people who use that custom element within React apps could self-organize a central repository for housing that integration file.

I think the possibility of such centralization is preferable to a solution that requires every user of the custom element to always have to be explicit with a sigil.

[LeeCheneler]

Option 3 would be my preferred but that's a huge breaking change... What about the inverse? Attributes have a prefix not props?

[robdodson]

@LeeCheneler

Option 3 would be my preferred but that's a huge breaking change... What about the inverse? Attributes have a prefix not props?

Why would it be a breaking change? The current behavior of attributes being the default would remain. The sigil would be opt-in and developers would use it to replace the spots in their code where they currently use a ref to pass data to a custom element as a JS property.

@drcmda

neither new attributes that could break existing projects.

Can you clarify what you meant by this?

[robdodson]

FYI for anyone following the discussion, I've updated the RFC with a 5th option suggested by members of the React team.

[gaearon]

Option 5 seems safest for us. It lets us add the feature without having to make a decision about “implicit” API right now since the ecosystem is still in the “figuring it out” phase. We can always revisit it in a few years.

Polymer’s paper-input element has 37 properties, so it would produce a very large config. If developers are using a lot of custom elements in their app, that may equal a lot of configs they need to write.

My impression is that custom element users in React will eventually want to wrap some custom elements into React components anyway for app-specific behavior/customizations. It is a nicer migration strategy for this case if everything already is a React component, e.g.

import XButton from './XButton';

and that happens to be generated by

export default ReactDOM.createCustomElementType(...)

This lets them replace a React component with a custom component that uses (or even doesn’t use) custom elements at any point in time.

So, if people are going to create React components at interop points, we might as well provide a powerful helper to do so. It is also likely that people will share those configs for custom elements they use.

And eventually, if we see the ecosystem stabilize, we can adopt a config-less approach.

I think the next step here would be to write a detailed proposal for how the config should look like to satisfy all common use cases. It should be compelling enough for custom element + React users, since if it doesn't answer common use cases (like event handling) we're going to end up in the limbo where the feature doesn't provide enough benefit to offset the verbosity.

[effulgentsia]

Building from my earlier comment, how about:

const XFoo = ReactDOM.createCustomElementType('x-foo', {
  propName1: {
    propertyName: string | null,
    attributeName: string | null,
    attributeSerializer: function | null,
    eventName: string | null,
  }
  propName2: {
  }
  ...
});

The logic would then be, for each React prop on an XFoo instance:

1. If the eventName for that prop is not null, register it as an event handler that invokes the prop value (assumed to be a function).
2. Else if rendering client-side and propertyName is not null, set the element property to the prop value.
3. Else if attributeName is not null, set the element attribute to the stringified prop value. If attributeSerializer is not null, use it to stringify the prop value. Otherwise, just do '' + propValue.

Polymer’s paper-input element has 37 properties, so it would produce a very large config.

I'd like to suggest that the config only be necessary for outlier props. For any prop on the XFoo instance that wasn't included in the config, default it to:

• if the value is a function:

eventName: the prop name,

• else:

propertyName: the prop name,
attributeName: camelCaseToDashCase(the prop name),

[effulgentsia]

Alternatively, maybe it makes sense to keep events in a separate namespace, in which case, remove everything having to do with eventName from the last comment, and instead let events be registered as:

<XFoo prop1={propValue1} prop2={propValue2} events={event1: functionFoo, event2: functionBar}>
</XFoo>

[robdodson]

@gaearon @effulgentsia what do y'all think of a combination of option 1 and option 5?

Option 1 would make it easier for the casual user of a custom element to pass rich data. I'm imagining the scenario where I'm building an app and I just want to use a couple of custom elements. I already know how they work and I'm not so invested that I want to write a config for them.

Option 5 would be for folks who want to use something like paper-input all over their app and would really like to expose its entire API to everyone on their team.

[robdodson]

For SSR of option 1 the heuristic could be always use an attribute if rendering on the server. A camelCase property gets converted to a dash-case attribute. That seems to be a pretty common pattern across web component libraries.

[effulgentsia]

I like the idea of an option1 + option5 combination a lot. Meaning that for most custom elements:

<x-foo prop1={propValue1}>

would work as expected: prop1 set as a property client-side and as a (dash-cased) attribute server-side.

And people could switch to option5 for anything for which the above doesn't suit them.

It would be a breaking change though from the way React 16 works. For anyone who experiences that breakage (e.g., they were using a custom element with attributes that aren't backed by properties), they could switch to option5, but it's still a break. I leave it to the React team to decide if that's acceptable.

[LeeCheneler]

Ah, this is what I get for reading this quickly on the train @robdodson 🤦‍♂️ ... Not really a fan of option 3 now 🤔 I read it as an all in on props being prefixed, hence my hesitation.

Option 5 seems reasonable and straightforward.

I like where @effulgentsia is heading. Is there a reason it couldn't be:

const XFoo = ReactDOM.createCustomElementType('x-foo', {
  propName1: T.Attribute,
  propName2: T.Event,
  propName3: T.Prop
})

Or is supporting multiple types on a single prop valuable?

I'd be hesitant with this flow though @effulgentsia:

if the value is a function:
eventName: the prop name,
else:
propertyName: the prop name,
attributeName: camelCaseToDashCase(the prop name),

I don't think I'd want a function prop to default to an event, and is assigning both propertyName and attributeName sensible? When would you want both supported to mimic the question above? 🙂

[effulgentsia]

@LeeCheneler:

Quoting from the issue summary's Option 1 pros:

Any element created with libraries like Polymer or Skate will automatically generate properties to back their exposed attributes. These elements should all "just work" with the above approach. Developers hand-authoring vanilla components are encouraged to back attributes with properties as that mirrors how modern (i.e. not oddballs like <input>) HTML5 elements (<video>, <audio>, etc.) have been implemented.

So that's the reason why assigning both propertyName and attributeName is sensible: because it reflects what is actually the case for elements that follow best practice. And by making React aware of that, it allows React to decide which to use based on situation: such as using properties for client-side rendering and attributes for server-side rendering. For custom elements that don't follow best practice and have some attributes without corresponding properties and/or some properties without corresponding attributes, React would need to be aware of that, so that attribute-less-properties aren't rendered during SSR and property-less-attributes can be set with setAttribute() during client-side rendering.

With your proposal, that could potentially be done by bit-combining flags, such as:

propName1: T.Property | T.Attribute,

However, that wouldn't provide a way to express that the attribute name is different from the property name (e.g., camelCase to dash-case). Nor would it provide a way to express how to serialize a rich object to an attribute during SSR (the current behavior of "[object Object]" isn't useful).

I don't think I'd want a function prop to default to an event

Yeah, I think I agree with that as well, hence the follow-up comment. Thanks for validating my hesitance with that!

[bjankord]

Any likelihood that this gets released as a stable feature before Oct 24, 2023?

[weihong1028]

Nice,thank you！                               ——weihong

[deeg05]

ngl i cant get over how cringe react is xdd

[weihong1028]

Nice,thank you！                               ——weihong

[appboot-platform]

Nice,thank you！                               ——weihong

Please don't post meaningless replies.

[weihong1028]

Nice,thank you！                               ——weihong

[trusktr]

As an addon to Option 2 (because a huristic like that can't be 100% successful all the time), and as a replacement to Option 3 without requiring new syntax, there should be a way to set either attribute or property when the uristic is not good enough.

Both JSX and html template tags in Solid.js allow this:

// with JSX:
const div = <div attr:foo={123} prop:bar={123}></div>
// with html template tag:
const div2 = html`<div attr:foo=${123} prop:bar=${123}></div>`

where attr:foo=${123} will set a foo attribute, and prop:bar=${123} will set a bar JS property.

What I'm saying is we could have Option 2 by default, otherwise if override with attr: or prop:.

Or it could be explicit. Lit element has no huristic, it just does what you say. In this case, that would be skipping Option 2, and going only with attr: and prop:.

Option 6

Let's call attr: and prop: Option 6, and consider coing with a combination of Option 2 and 6.

[jfbrennan]

@trusktr That seems like a foot-gun. Users of a custom element are now responsible for knowing its internals, i.e. I have to go learn if foo is only an attribute and be sure to attr:foo and never do prop:foo. If it's both, well now I'm left wondering if attr: or prop: would be better.

Property first, then attribute (option 2) solves all cases in a foolproof way (unless you can elaborate on "can't be 100% successful all the time", I'm not aware of edge cases that would break Option 2, genuinely curious to learn).

[bahrus]

@jfbrennan

Some edge cases to consider, and bringing you up to date on this tortuous debate:

1. As pointed out by @josepharhar, if you name a property beginning with the word "on", React could confuse it with an event.
2. For years, some members of the React team claimed web components were fundamentally flawed, because they encouraged developers to define attribute names without dashes, which could conflict with future platform attributes, and that is why they were dragging their feet on supporting web components. Yet by React opting for option 2 (yes, they did endorse option 2 many comments and months ago), they essentially shot their own argument in the foot (speaking of footguns) -- if a developer chooses to use dashes in their attribute names to address this very concern, say on the server side for setting initial values, they will seemingly run into trouble on the client side, if they utilize the same React template for both scenarios.
3. It's unclear how making all bindings work only with properties would make React work with solutions like HTMX or Alpine.js or other such solutions, at least if they need the values of those attributes to change dynamically.

At this point, I almost wonder if just leaving things the way they are wouldn't be better (at least compared to option 2). It provides quite a testimony to React's engineering prowess, and/or their stance on issues like these.

In the meantime, it has helped propel initiatives like the custom element manifest, which I think also showcases the engineering prowess and commitment to the same vendor lock-in issue mentioned above, but within the web component community.

Most libraries, like Shoelace simply document separately how to integrate with React. If I were a member of the React team, I would find this quite embarrassing, personally, so I've grown rather fond of seeing these "accommodations" for the special framework that is React. (On the other hand, I'm not the one who is having to write such documentation, so my flippant attitude is probably not shared by anyone else, so, yes, by all means proceed with option 2 (sigh)).

[jfbrennan]

@bahrus thanks for those insights. I'm glad to see more people and projects holding React's feet to the fire when it comes to Web Components - it is indeed React that needs to figure out how to play nice with WC, not the other way around. My preference is actually Option 0 where React ultimately fails to ship a good WC solution so we can then justify rewrites with simple, standards-compliant frameworks and never ever try to outsmart the web platform again 😇

[andyearnshaw]

@gaearon

Once the flag is removed, barring any extraordinary circumstances, it should ship in the next major.

Do you know if there will be a canary release before then?

[sophiebits]

@andyearnshaw Once #27450 lands (likely any day now), the next canary release will include the changes.

[jakearchibald]

Is there a summary of the adopted behaviour anywhere? Is it one of the options in the OP?

[frehner]

@jakearchibald AFAIK this PR #22184 is the base of what was implemented; @josepharhar outlined the implementation in this comment #22184 (comment)

[jfbrennan]

Not finding a useful summary in that PR or other linked GH issues.

Two things I saw that did get my attention but wasn't immediately clear what the decision was:

1. Regarding event handler attributes:

I originally proposed that we do the same thing as preact here (forward everything starting with "on" to addEventListener), but if I remember correctly Sebastian felt strongly that we shouldn't reserve everything starting with "on" for event listeners. Jason Miller also felt strongly about this the other way, so maybe it's something that I can revisit/discuss more after merging this PR - maybe more users will chime in if they get to test it out.

Is this an argument React should even be having? The standard says: "the event handler [content attribute or IDL] is exposed through a name, which is a string that always starts with "on" and is followed by the name of the event for which the handler is intended."
There are no non-event HTML attributes that start with "on" for this reason. Web Component authors must follow this, including custom attributes and custom events, so React simply needs to follow the spec here.

2. In the linked Google doc this doesn't sound like the right design:

we avoid showing the custom element until it is defined

The Custom Element definition may never come (e.g. web server is down, network is slow/stalled, developer failed to include script), but browsers today can and do immediately render yet-to-be-defined custom elements in the DOM because they're designed to be fast and fault tolerant.

Not only that, but Custom Element authors don't always want/need to use shadow DOM, so browsers can and do immediately begin to style custom elements with global styles. Again, the CE definition may still be on its way or a multitude of errors prevent it from ever downloading/evaluating, but that's not a reason to halt rendering what can be rendered.

Lastly, and I'm biased here, but I personally use all of this to my advantage (TAC CSS methodology, Mdash).
For example, basic custom elements like an icon or badge have no need for JavaScript, so I don't create CE definitions for them. But the API of custom elements (i.e. tag and attributes) is better than a bunch of goofy classes, so this is what I do:

Icon component with custom HTML tag, it's 100% vanilla CSS, and no CE definition

/* Icon base styles */
m-icon {
  display: inline-flex;

  &::before {
    font-family: 'm-icons'; /* Mdash uses Material Symbols Outlined with filled option */
    content: attr(name);
    -webkit-font-smoothing: antialiased;
  }

  /* Filled option */
  &[fill]::before { font-variation-settings: 'FILL' 1 }
}

And developers use it just like HTML because it is HTML:

<p>Material Symbols star icon <m-icon name="star"></m-icon></p>
<p>Material Symbols filled star icon <m-icon name="star" fill></m-icon></p>

Works literally everywhere. Been doing it for 5+ years now in dozens of projects - static HTML, SSR, CSR, with frameworks including React and without - and it's been great. Sometimes these CSS-only elements get new requirements that have to use JavaScript and then I add a CE definition for that behavior, but the markup and styles do not change. It's like progressive enhancement for Custom Elements.

So, please do not break this by waiting for CE to be defined in order to render! Custom HTML like this has been possible for decades and is supported by all browsers for almost just as long. React should follow the browser and every other framework and render what is given to it. Let the CE definition do whatever it does if and when it exists.

[jakearchibald]

There are no non-event HTML attributes that start with "on" for this reason. Web Component authors must follow this

They 'should' follow this, but there's nothing forcing them. Have you looked at the tests in the PR that shows React's behaviour? It seems pretty reasonable.

So, please do not break this by waiting for CE to be defined in order to render!

Agreed. CSS can provide a nice 'before defined' rendering of a custom element, especially in combination with declarative shadow DOM.

[msteller-connyun]

React is the only of the major frameworks which needs an "on" for events.
Existing web component component libraries also don't have an on in their signature:
https://shoelace.style/components/button#events
And I'm pretty sure they will not add an "on" to all their event names because of React.

Or are we talking about if React adds this prefix automatically for web components?
So you would write <sl-button onSlBlur="..."> but it would call the event sl-blur?

[jakearchibald]

I believe the event will be SLBlur. You'd use onsl-blur if you wanted the event sl-blur.

[jfbrennan]

@msteller-connyun nothing really to do with React. The event names themselves - native and custom - don't include the "on" prefix. Shoelace event names, Mdash event names, etc. don't include "on" in their event names. The spec defines event handler attribute names start with an "on" prefix "followed by the name of the event". So, using whatever framework (or no framework) when binding events in the template you use the event name prefixed with the framework syntax:

Event name: "slblur"

HTML
<sl-button onslblur="...">

Riot (uses vanilla HTML syntax)
<sl-button onslblur="...">

Vue
<sl-button v-on:slblur="...">
<sl-button @slblur="...">

Svelte
<sl-button on:slblur="...">

React
<sl-button onslblur="...">

[WickyNilliams]

React won't be changing the case at all, it'll use the event name verbatim. Since event names are case sensitive, and react can't know the casing of an event from a custom element, the only safe option is to listen exactly to everything after on.

oneventName -> eventName
onEventName -> EventName 
onevent-name -> event-name
oneventname -> eventname

So in the case of the shoelace example it'll be onsl-blur

At least this was the case when I lasted tested the experimental build

[jfbrennan]

Thanks for pointing to the tests @jakearchibald.
Looks great, custom boolean attribute seems to be missing from the SSR test though.

[justinfagnani]

Not that this would not work:

HTML
<sl-button onslblur="...">

You need to call addEventListener('sl-blur').

The on... syntax only works if the element specifically listens for an attribute name onslblur, calls addEventListener() on itself, and evals the string when events fire.

[jakearchibald]

@WickyNilliams you are right. I updated my comment which wrongly assumed the event names were lowercased. Here's a relevant test https://github.com/facebook/react/pull/22184/files#diff-db98d094dc4f5b1a9c32b4da4df310370707408a929762868d44bf1acd515923R174

[bjankord]

Wild that this issue is 6 years old. Still hoping to see the solution from the experimental branch make its way into the stable release soon.

[jfbrennan]

@bjankord 6 years and counting to make the world's most popular web framework compatible with the web. It's Internet Explorer all over again and it's not just WC either #17157

[rickhanlonii]

Plan for custom elements support in next stable release

Update: The plan is to release this in React 19, which will have direct support for custom elements. Developers can expect that all tests on custom-elements-everywhere.com will pass by default going forward, like they currently do in the experimental channels. Release date, as well as docs for what's supported, still to be announced.

For information on how it's implemented see @josepharhar's PR implementing the proposal here: #22184

[rickhanlonii]

Update: Custom element support has landed in Canary

#27450 has landed, which moves custom element support from experimental to canary (note: as of the time of this comment, the canary hasn't published yet, but it will be published automatically in the next day).

This means it will land in the next stable release of React: React 19 🎉

We will include docs for the behavior with the React 19 announcements. Until then, you can read implemented behavior here.