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Upgrading from Preact 8.x

This document is intended to guide you through upgrading an existing Preact 8.x application to Preact X and is divided in 3 main sections

Preact X brings many new exciting features such as Fragments, hooks and much improved compatibility with the React ecosystem. We tried to keep any breaking changes to the minimum possible, but couldn't eliminate all of them completely without compromising on our feature set.

Upgrading dependencies

Note: Throughout this guide we'll be using the npm client and the commands should be easily applicable to other package managers such as yarn.

Let's begin! First install Preact X:

npm install preact

Because compat has moved to core, there is no need for preact-compat anymore. Remove it with:

npm remove preact-compat

To guarantee a stable ecosystem for our users (especially for our enterprise users) we've released major version updates to Preact X related libraries. If you're using preact-render-to-string you need to update it to the version that works with X.

Library	Preact 8.x	Preact X
`preact-render-to-string`	4.x	5.x
`preact-router`	2.x	3.x
`preact-jsx-chai`	2.x	3.x
`preact-markup`	1.x	2.x

Compat has moved to core

To make third-party React libraries work with Preact we ship a compatibility layer that can be imported via preact/compat. It was previously available as a separate package, but to make coordination easier we've moved it into the core repository. So you'll need to change existing import or alias declarations from preact-compat to preact/compat (note the slash).

Be careful not to introduce any spelling errors here. A common one seems to be to write compact instead of compat. If you're having trouble with that, think of compat as the compatibility layer for react. That's where the name is coming from.

Third party libraries

Due to the nature of the breaking changes, some existing libraries may cease to work with X. Most of them have been updated already following our beta schedule but you may encounter one where this is not the case.

preact-redux

preact-redux is one of such libraries that hasn't been updated yet. The good news is that preact/compat is much more React-compliant and works out of the box with the React bindings called react-redux. Switching to it will resolve the situation. Make sure that you've aliased react and react-dom to preact/compat in your bundler.

Remove preact-redux
Install react-redux

mobx-preact

Due to our increased compatibility with the react-ecosystem this package isn't needed anymore. Use mobx-react instead.

Remove mobx-preact
Install mobx-react

styled-components

Preact 8.x only worked up to styled-components@3.x. With Preact X this barrier is no more and we work with the latest version of styled-components. Make sure that you've aliased react to preact correctly.

preact-portal

The Portal component is now part of preact/compat.

Remove preact-portal
Import createPortal from preact/compat

Getting your code ready

Using named exports

To better support tree-shaking we don't ship with a default export in preact core anymore. The advantage of this approach is that only the code you need will be included in your bundle.

// Preact 8.x
import Preact from "preact";

// Preact X
import * as preact from "preact";

// Preferred: Named exports (works in 8.x and Preact X)
import { h, Component } from "preact";

Note: This change doesn't affect preact/compat. It still has both named and a default export to remain compatible with react.

`render()` always diffs existing children

In Preact 8.x, the calls to render() would always append the elements to the container.

// Existing markup:
<body>
  <div>hello</div>
</body>

render(<p>foo</p>, document.body);
render(<p>bar</p>, document.body);

// Preact 8.x output:
<body>
  <div>hello</div>
  <p>foo</p>
  <p>bar</p>
</body>

In order to diff existing children in Preact 8, an existing DOM node had to be provided.

// Existing markup:
<body>
  <div>hello</div>
</body>

let element;
element = render(<p>foo</p>, document.body);
element = render(<p>bar</p>, document.body, element);

// Preact 8.x output:
<body>
  <div>hello</div>
  <p>bar</p>
</body>

In Preact X, render() always diffs DOM children inside of the container. So if your container contains DOM that was not rendered by Preact, Preact will try to diff it with the elements you pass it. This new behavior more closely matches the behavior of other VDOM libraries.

// Existing markup:
<body>
  <div>hello</div>
</body>

render(<p>foo</p>, document.body);
render(<p>bar</p>, document.body);

// Preact X output:
<body>
  <p>bar</p>
  <div>hello</div>
</body>

If you are looking for behavior that exactly matches how React's render method works, use the render method exported by preact/compat.

`props.children` is not always an `array`

In Preact X we can't guarantee props.children to always be of type array anymore. This change was necessary to resolve parsing ambiguities in regards to Fragments and components that return an array of children. In most cases you may not even notice it. Only in places where you'll use array methods on props.children directly need to be wrapped with toChildArray. This function will always return an array.

// Preact 8.x
function Foo(props) {
  // `.length` is an array method. In Preact X when `props.children` is not an
  // array, this line will throw an exception
  const count = props.children.length;
  return <div>I have {count} children </div>;
}

// Preact X
import { toChildArray } from "preact";

function Foo(props) {
  const count = toChildArray(props.children).length;
  return <div>I have {count} children </div>;
}

Don't access `this.state` synchronously

In Preact X the state of a component will no longer be mutated synchronously. This means that reading from this.state right after a setState call will return the previous values. Instead you should use a callback function to modify state that depends on the previous values.

this.state = { counter: 0 };

// Preact 8.x
this.setState({ counter: this.state.counter + 1 });

// Preact X
this.setState(prevState => {
  // Alternatively return `null` here to abort the state update
  return { counter: prevState.counter + 1 };
});

`dangerouslySetInnerHTML` will skip diffing of children

When a vnode has the property dangerouslySetInnerHTML set Preact will skip diffing the vnode's children.

<div dangerouslySetInnerHTML="foo">
  <span>I will be skipped</span>
  <p>So will I</p>
</div>

Notes for library authors

This section is intended for library authors who are maintaining packages to be used with Preact X. You can safely skip this section if you're not writing one.

The `VNode` shape has changed

We renamed/moved the following properties:

attributes -> props
nodeName -> type
children -> props.children

As much as we tried, we always ran into edge-cases with third-party libraries written for react. This change to our vnode shape removed many difficult to spot bugs and makes our compat code a lot cleaner.

Adjacent text nodes are not joined anymore

In Preact 8.x we had this feature where we would join adjacent text notes as an optimization. This doesn't hold true for X anymore because we're not diffing directly against the dom anymore. In fact we noticed that it hurt performance in X which is why we removed it. Take the following example:

// Preact 8.x
console.log(<div>foo{"bar"}</div>);
// Logs a structure like this:
//   div
//     text

// Preact X
console.log(<div>foo{"bar"}</div>);
// Logs a structure like this:
//   div
//     text
//     text