Custom Elements Interop changes breaking

Overview

BREAKING: Custom elements whitelisting is now performed during template compilation, and should be configured via compiler options instead of runtime config.
BREAKING: Special is prop usage is restricted to the reserved <component> tag only.
NEW: There is new v-is directive to support 2.x use cases where is was used on native elements to work around native HTML parsing restrictions.

Autonomous Custom Elements

If we want to add a custom element defined outside of Vue (e.g. using the Web Components API), we need to ‘instruct’ Vue to treat it as a custom element. Let’s use the following template as an example.

<plastic-button></plastic-button>

2.x Syntax

In Vue 2.x, whitelisting tags as custom elements was done via Vue.config.ignoredElements:

// This will make Vue ignore custom element defined outside of Vue
// (e.g., using the Web Components APIs)
Vue.config.ignoredElements = ['plastic-button']

3.x Syntax

In Vue 3.0, this check is performed during template compilation. To instruct the compiler to treat <plastic-button> as a custom element:

If using a build step: pass the isCustomElement option to the Vue template compiler. If using vue-loader, this should be passed via vue-loader‘s compilerOptions option:

// in webpack config
rules: [
  {
    test: /\.vue$/,
    use: 'vue-loader',
    options: {
      compilerOptions: {
        isCustomElement: tag => tag === 'plastic-button'
      }
    }
  }
  // ...
]

If using on-the-fly template compilation, pass it via app.config.isCustomElement:
```
const app = Vue.createApp({})
app.config.isCustomElement = tag => tag === 'plastic-button'
```
It’s important to note the runtime config only affects runtime template compilation - it won’t affect pre-compiled templates.

Customized Built-in Elements

The Custom Elements specification provides a way to use custom elements as Customized Built-in Element by adding the is attribute to a built-in element:

<button is="plastic-button">Click Me!</button>

Vue’s usage of the is special prop was simulating what the native attribute does before it was made universally available in browsers. However, in 2.x it was interpreted as rendering a Vue component with the name plastic-button. This blocks the native usage of Customized Built-in Element mentioned above.

In 3.0, we are limiting Vue’s special treatment of the is prop to the <component> tag only.

When used on the reserved <component> tag, it will behave exactly the same as in 2.x;
When used on normal components, it will behave like a normal prop:
```
<foo is="bar" />
```
- 2.x behavior: renders the bar component.
- 3.x behavior: renders the foo component and passing the is prop.
When used on plain elements, it will be passed to the createElement call as the is option, and also rendered as a native attribute. This supports the usage of customized built-in elements.
```
<button is="plastic-button">Click Me!</button>
```
- 2.x behavior: renders the plastic-button component.
- 3.x behavior: renders a native button by calling
```
document.createElement('button', { is: 'plastic-button' })
```

v-is for In-DOM Template Parsing Workarounds

Note: this section only affects cases where Vue templates are directly written in the page’s HTML. When using in-DOM templates, the template is subject to native HTML parsing rules. Some HTML elements, such as <ul>, <ol>, <table> and <select> have restrictions on what elements can appear inside them, and some elements such as <li>, <tr>, and <option> can only appear inside certain other elements.

2x Syntax

In Vue 2 we recommended working around with these restrictions by using the is prop on a native tag:

<table>
  <tr is="blog-post-row"></tr>
</table>

3.x Syntax

With the behavior change of is, we introduce a new directive v-is for working around these cases:

<table>
  <tr v-is="'blog-post-row'"></tr>
</table>

WARNING

v-is functions like a dynamic 2.x :is binding - so to render a component by its registered name, its value should be a JavaScript string literal:

<!-- Incorrect, nothing will be rendered -->
<tr v-is="blog-post-row"></tr>
<!-- Correct -->
<tr v-is="'blog-post-row'"></tr>

Migration Strategy

Replace config.ignoredElements with either vue-loader‘s compilerOptions (with the build step) or app.config.isCustomElement (with on-the-fly template compilation)
Change all non-<component> tags with is usage to <component is="..."> (for SFC templates) or v-is (for in-DOM templates).