
Lightning Web Runtime, or LWR for short was released back in [Spring 21 (v51.0)](https://help.salesforce.com/s/articleView?id=release-notes.rn_experiences_developers_lwr.htm&release=230&type=5). Even before the release of LWR, most developers dreamed of being able to build custom nested components. We could see that OOTB (Out Of The Box) components could use them (see Tabs component).

Salesforce recently released the ability to build custom LWCs with child `<slot/>`'s, to allow nesting of components. Nested LWCs allow you to compose multiple components together into a single component. The child components maintain their own life cycle and can communicate with each other through events, making it easy to create flexible and dynamic apps.

W> Before continuing, Aura based Communities are (sadly) not supported. So if you're currently using them it might be time to make the switch to LWR.

## Code Example

Enough talk, lets jump into the code and see the magic happen.

### HTML

We'll start by creating the HTML template to hold our `<slot/>`'s.

```html
<template>
  <template if:false={isAura}>
    <div class="container">
      <div>
        <slot name="region1"></slot>
      </div>
      <div>
        <slot name="region2"></slot>
      </div>
      <div>
        <slot name="region3"></slot>
      </div>
    </div>
  </template>
</template>
```

### JavaScript

Now lets have a look at the JavaScript. The important part (or magic) here is the `@slot` definitions in the comment directly above the LWC export. Without these definitions, the dynamic components will not work.

The `isAura` check also ensures this component shows a warning in the experience builder when it is being used from within an Aura based community.

```javascript
import { LightningElement } from 'lwc'

/**
 * @slot region1
 * @slot region2
 * @slot region3
 */
export default class NestedLWCs extends LightningElement {
  get isAura() {
    return window['$A'] !== undefined && window['$A'] !== null
  }
}
```

### Metadata XML

Make sure your `.xml` file is setup to be available in the experience builder. The key values are setting `isExposed` to `true` and including both `lightningCommunity__Page` and `lightningCommunity__Default` as `targets`.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
  <apiVersion>57.0</apiVersion>
  <isExposed>true</isExposed>
  <masterLabel>Nested LWCs</masterLabel>
  <description>Nested LWC magic</description>
  <targets>
    <target>lightningCommunity__Page</target>
    <target>lightningCommunity__Default</target>
  </targets>
</LightningComponentBundle>
```

## Experience Builder

Now lets open up the Experience Builder and see our custom LWC in action.

![Experience Builder: Draging our custom LWC onto the page](/lwc-nested-components/nested-lwc-1.gif)

We can also nest components within other nested components. I am not aware of any limitation here. I was able to place up to 10 levels deep without any issues, noting theses are very barebones test LWCs. More complex LWCs may start affecting performance.

![Experience Builder: Draging our custom LWC onto the page](/lwc-nested-components/nested-lwc-2.gif)

## Wrapping Up

In summary, nested LWCs provide a powerful and flexible way for developers to create custom components that can be composed of multiple other components, making it easier to build dynamic and reusable apps on the Experience Cloud platform.


A quick look into how to build LWCs with slots to allow for nesting of LWCs

Lightning Web Components: Custom Nested Components


Mixins (or Abstract subclasses) are templates for classes that allow us to write generic functionality to be reused. Given the nature of classes, it is not possible to have more than one superclass. Mixins allow us to pass the base class as a property and unlock nested inheritance.

## JavaScript Mixin Usage

Before we dive into an LWC example, lets look at how we can use mixins in a plain javascript environment.

Firstly, we must create a mixin function that our classes can use. This should be a piece of generic functionality that multiple classes can utalise.

```javascript
// A simple Mixin method
// adds the `windowSize()` method to classes that implement this mixin
export const WindowSizeMixin = (BaseClass) =>
  class extends BaseClass {
    get windowSize() {
      return {
        width: window.innerWidth,
        height: window.innerHeight,
      }
    }
  }
```

```javascript
import { WindowSizeMixin } from './windowSizeMixin'

// Implementing the `WindowSizeMixin` gives us access to the
// `this.windowSize` getter
class Header extends WindowSizeMixin(BaseClass) {
  getWindowSize() {
    return this.windowSize
  }
}
```

In the above example, calling the `getWindowSize()` function on our Header class will return the object defined in the `WindowSizeMixin`.

## LWC Mixin Usage

Now we have a basic understanding of how mixins work in JavaScript, lets look at how the concept applies to LWC. Using mixins in LWC is almost identical with one difference, your `BaseClass` should be `LightningElement` (or another class that has extended `LightningElement`).

We'll assume the same above mixin source code that we used above (`WindowSizeMixin`) but instead our implementation will be a LWC component.

```javascript
// don't forget to import LightningElement, we need to extend it as our base class
import { LightningElement } from 'lwc'

// assume our mixin has been created as a separate LWC to be used by multiple components
import { WindowSizeMixin } from 'c/windowSizeMixin'

// LWC expects a default export of our `LightingElement` extended component
export default class Header extends WindowSizeMixin(LightningElement) {
  getWindowSize() {
    // available via the WindowSizeMixin
    return this.windowSize
  }
}
```

## Stacking Mixins

The beauty of mixins is that you can use more than one. This can be useful for creating smaller snippet mixins for similar functionality that is used across multiple LWCs.

Lets have a quick look at an example. We will create another mixin method.

```javascript
export const DateMixin = (BaseClass) =>
  class extends BaseClass {
    // a function that returns the current date as an object
    getDate() {
      const date = new Date()
      return {
        day: date.getDate(),
        month: date.getMonth() + 1,
        year: date.getFullYear(),
      }
    }
  }
```

Lets update our LWC example from above to include the new `DateMixin`.

```javascript
// don't forget to import LightningElement, we need to extend it as our base class
import { LightningElement } from 'lwc'

// assume our mixins have been created as a separate LWC to be used by multiple components
import { WindowSizeMixin } from 'c/windowSizeMixin'
import { DateMixin } from 'c/dateMixin'

// LWC expects a default export of our component
// we can stack mixins here
export default class Header extends DateMixin(
  WindowSizeMixin(LightningElement)
) {
  getWindowSize() {
    // available via the WindowSizeMixin
    return this.windowSize
  }
  getCurrentDate() {
    // available via the DateMixin
    return this.getDate()
  }
}
```

## Using `@wire` in a Custom Mixin

[`@wire`](https://developer.salesforce.com/docs/component-library/documentation/en/lwc/lwc.data_wire_service_about) is a service that streams immutable data to the component. This is heavily used in on platform development and a must when it comes to building dynamic components.

Lets have a look at a simple example mixin that uses the standard [`getRecord`](https://developer.salesforce.com/docs/component-library/documentation/en/lwc/reference_wire_adapters_record) wire method.

```javascript
import { wire } from 'lwc'
import { getRecord } from 'lightning/uiRecordApi'

export const AccountWireMixin = (BaseClass) =>
  class extends BaseClass {
    // explicitly not setting recordId so that the consuming
    // component can choose how it wishes to implement it (e.g @api, get/set)
    // recordId

    // this can be dynamically changed by the implementing class if needed
    // this array acts as a default field list
    fieldRef = ['Account.Id', 'Account.Name', 'Account.Phone']

    @wire(getRecord, {
      recordId: '$recordId',
      fields: '$fieldRef',
    })
    wiredGetRecord(response) {
      // you can choose to either set the data to a variable for you component to consume
      // or fire off a function that your consuming component can override.
      // for this example, we will call a method
      this.handleWiredAccount(response)
    }

    /**
     * This method should be overriden by the consuming component
     */
    handleWiredAccount = (response) => {
      const { error, data } = response

      if (error) {
        // TODO: handle errors
      } else if (data) {
        // TODO: handle success
      }
    }
  }
```

We can update our above example and stack the `AccountWireMixin` on top.

```javascript
// don't forget to import LightningElement, we need to extend it as our base class
import { LightningElement } from 'lwc'

// assume our mixins have been created as a separate LWC to be used by multiple components
import { WindowSizeMixin } from 'c/windowSizeMixin'
import { DateMixin } from 'c/dateMixin'
import { AccountWireMixin } from 'c/accountWireMixin'

// LWC expects a default export of our component
// we can stack mixins here
export default class Header extends AccountWireMixin(
  DateMixin(WindowSizeMixin(LightningElement))
) {
  // ... existing code

  // override the shell method defined in the AccountWireMixin
  handleWiredAccount = (response) => {
    const { error, data } = response

    if (error) {
      // TODO: handle errors
      console.error('Oh no, something went wrong ', error)
    } else if (data) {
      // TODO: handle success
      console.log('We have account data, yay ', data)
    }
  }
}
```

## Wrapping up

In conclusion, mixins are an incredibly useful tool in LWC development. They allow us to write generic, reusable functionality that can be shared between multiple components. By stacking multiple mixins, we are able to compose complex components from smaller, focused mixins. Mixins help keep our code DRY (Don't Repeat Yourself) and maintainable.


A short guide on how to implement your own custom mixins in LWC

lwc

Articles

Lightning Web Components: Custom Nested Components

Lightning Web Components: Custom Mixins