Component Composition in LWC

You can add components within the body of another component. Component Composition in LWC enables you to build complex components from simpler building-block components. Composing apps and components from a set of smaller components makes code more reusable and maintainable.

Let’s look at a simple app. The components are in the namespace. The markup is contrived because we want to illustrate the concepts of owner and container.

<!-- todoApp.html -->
        <example-todo-item item-name="Milk"></example-todo-item>
        <example-todo-item item-name="Bread"></example-todo-item>


The owner is the component that owns the template. In this example, the owner is the example-todo-app component. The owner controls all the composed (child) components that it contains. An owner can:

  • Set public properties on composed components
  • Call public methods on composed components
  • Listen for events dispatched by composed components


A container contains other components but itself is contained within the owner component. In this example, example-todo-wrapper is a container. A container is less powerful than the owner. A container can:

  • Read, but not change, public properties in contained components
  • Call public methods on composed components
  • Listen for some, but not necessarily all, events bubbled up by components that it contains.

Parent and child

When a component contains another component, which, in turn, can contain other components, we have a containment hierarchy. In the documentation, we sometimes talk about parent and child components. A parent component contains a child component. A parent component can be the owner or a container.

Set a Property on a Child Component

To communicate down the containment hierarchy, an owner can set a property on a child component. An attribute in HTML turns into a property assignment in JavaScript.

Let’s look at how the owner, example-todo-app, sets public properties on the two instances of example-todo-item.

In the playground, select todoItem.js. The @api decorator exposes the itemName field as a public property.

Select todoApp.html. To set the public itemName property, it sets the item-name attribute on each example-todo-item component. Change Milk to Hummus to see an item name change.

import { LightningElement, api } from 'lwc';
export default class TodoApp extends LightningElement {}


    <example-todo-item item-name="Milk"></example-todo-item>
    <example-todo-item item-name="Bread"></example-todo-item>


import { LightningElement, api } from 'lwc';
export default class TodoItem extends LightningElement {
    @api itemName;



Property names in JavaScript are in camel case while HTML attribute names are in kebab case (dash-separated) to match HTML standards. In todoApp.html, the item-name attribute in markup maps to the itemName JavaScript property of todoItem.js.

This example uses the static values Milk and Bread. A real-world component would typically use a for:each iteration over a collection computed in the owner’s JavaScript file, todoApp.js.

Send an Event from a Child to an Owner

When a component decorates a field with @api to expose it as a public property, it should set the value only when it initializes the field, if at all. After the field is initialized, only the owner component should set the value.

To trigger a mutation for a property value provided by an owner component, a child component can send an event to the owner. The owner can change the property value, which propagates down to the child.


The Lightning Web Components recipes app has a more realistic example. Click Child-to-Parent and look at the EventWithData example, which has a list of contacts. When a user clicks a contact, the component sends an event to the parent, which sends a new contact object to the child.

To prevent code complexity and unexpected side effects, data should flow in one direction, from owner to child.

Primitive Property Values

We recommend using primitive data types for public properties instead of using object data types. Slice complex data structures in a higher-level component and pass the primitive values to the component descendants.

Primitive values require specific @api properties that clearly define the data shape. Accepting an object or an array requires documentation to specify the shape. If an object shape changes, consumers break.

Standard HTML elements accept only primitive values for attributes. When a standard HTML element needs a complex shape, it uses child components. For example, a table element uses tr and td elements. Only primitive types can be defined in HTML. For example, <table data={...}> isn’t a value in HTML. However, you could create a table Lightning web component with a data API.

Objects Passed to Components Are Read-Only

A non-primitive value (like an object or array) passed to a component is read-only. The component cannot change the content of the object or array. If the component tries to change the content, you see an error in the browser console.

To mutate the data, the component can make a shallow copy of the objects it wants to mutate.

More commonly, the component can send an event to the owner of the object. When the owner receives the event, it mutates the data, which is sent back down to the child.

In the following playground, example-contact-tile-object exposes its contact field as a public property. The parent, example-composition-basics-object, defines a contact object and passes it to example-contact-tile-object. The child component can’t mutate the object.

  1. Open the browser’s JavaScript console and clear it.
  2. Click Update The contact-tile-object component tries and fails to update the name property of the object that it received from its owner.


The browser console displays the error: Uncaught Error: Invalid mutation: Cannot set "name" on "[object Object]". "[object Object]" is read-only. The component doesn’t own the object passed to it and can’t mutate it.

This is a tricky but important thing to understand. The contact object passed to example-contact-tile-object is read-only. That object is owned by the component that passed it to the child. However, the child owns its contact field and can set that field to a new value. Let’s look at some code to see how this works.

  1. Click Update contact field. The contact-tile-object component successfully sets a new value for the contact field.

Again, the contact-tile-object component owns its contact field, and it can assign a new value to that field.


After clicking Update contact field, the child component owns the object, which means that the child can mutate the object. Therefore, if you click Update again, you may expect the component to display Jennifer Wu, but it doesn’t. The value is set, and Jennifer Wu displays in the console, which means that the component did mutate the object. However, the component doesn’t rerender. Why? Because LWC can’t see that a property of the contact object changed. The contact field is decorated with @api, not @track. (A field can have only one decorator.) The @api decorator simply makes a field public and allows its value to be set from the outside. The @api decorator doesn’t tell LWC to observe mutations like @track does.

But why does the component rerender when we assign a new value directly to the contact field? Because LWC observes all fields for mutation. If you assign a new value to a field, the component rerenders. However, if an object is assigned to a field, LWC doesn’t observe the internal properties of that object. Therefore, the component doesn’t update when you assign a new value to To tell LWC to observe the internal properties of an object, decorate the field with @track

Call a Method on a Child Component

To expose a public method, decorate it with @api. Public methods are part of a component’s API. To communicate down the containment hierarchy, owner and parent components can call JavaScript methods (or set properties) on child components.


See the Parent-to-Child recipes in the Lightning Web Components recipes app.

Define a Method

This example exposes isPlaying(), play(), and pause() methods in a example-video-player component by adding the @api decorator to the methods. A parent component that contains example-video-player can call these methods. Here’s the JavaScript file.

videoUrl is a public reactive property. The @api decorator can be used to define a public reactive property, and a public JavaScript method, on a component. Public reactive properties are another part of the component’s public API.


To access elements that the template owns, the code uses the template property.

Now, let’s look at the HTML file where the video element is defined.

In a real-world component, example-video-player would typically have controls to play or pause the video itself. For this example to illustrate the design of a public API, the controls are in the parent component that calls the public methods.

Call a Method

The example-method-caller component contains example-video-player and has buttons to call the play() and pause() methods in example-video-player. Here’s the HTML.

Clicking the buttons in example-method-caller plays or pauses the video in example-video-player after we wire up the handlePlay and handlePause methods in example-method-caller.

Here’s the JavaScript file for example-method-caller.

The handlePlay() function in example-method-caller calls the play() method in the example-video-player element. this.template.querySelector('example-video-player') returns the example-video-player element in methodCaller.html. The this.template.querySelector() call is useful to get access to a child component so that you can call a method on the component.

The handlePause() function in example-method-caller calls the pause() method in the example-video-player element.

Return Values

To return a value from a JavaScript method, use the return statement. For example, see the isPlaying() method in example-video-player.

Method Parameters

To pass data to a JavaScript method, define one or more parameters for the method. For example, you could define the play() method to take a speed parameter that controls the video playback speed.Pass Markup into Slots

Add a slot to a component’s HTML file so a parent component can pass markup into the component. A component can have zero or more slots.

A slot is a placeholder for markup that a parent component passes into a component’s body. Slots are part of the Web Component specification.

To define a slot in markup, use the <slot> tag, which has an optional name attribute.

Unnamed Slots

In the playground, click slotDemo.html to see an unnamed slot. The slotWrapper component passes content into the slot.

When example-slot-demo is rendered, the unnamed slot is replaced with Content from Slot Wrapper. Here’s the rendered HTML of example-slot-wrapper.

If a component has more than one unnamed slot, the markup passed into the body of the component is inserted into all the unnamed slots. This UI pattern is unusual. A component usually has zero or one unnamed slot.

Named Slots

This example component has two named slots and one unnamed slot.

Here’s the markup for a parent component that uses example-named-slots.

The example-slots-wrapper component passes:

  • Willy into the firstName slot
  • Wonka into the lastName slot
  • Chocolatier into the unnamed slot

Here’s the rendered output.

Run Code on slotchange

All <slot> elements support the slotchange event. The slotchange event fires when a direct child of a node in a <slot> element changes, such as when new content is appended or deleted. Only <slot> elements support this event.

Changes within the children of the <slot> element don’t trigger a slotchange event.

This example contains a <slot> element that handles the slotchange event.

The component example-child is passed into the slot.

The console prints the first time the component is rendered, and if the flag addOneMore is set to true.

The slotchange event is not triggered even when showFooter is true and the footer element is appended. Query Selectors

The querySelector() and querySelectorAll() methods are standard DOM APIs. querySelector() returns the first element that matches the selector. querySelectorAll() returns an array of DOM Elements.

Call these methods differently depending on whether you want to access elements the component owns or access elements passed via slots.

If you’re iterating over an array, consider adding some other attribute to the element, like a class or data-* value, and use it to select the element. LWC uses id values for accessibility only.

Access Elements the Component Owns

To access elements rendered by your component, use the template property to call a query method.

  • The order of elements is not guaranteed.
  • Elements not rendered to the DOM aren’t returned in the querySelector() result.
  • Don’t use ID selectors. The IDs that you define in HTML templates may be transformed into globally unique values when the template is rendered. If you use an ID selector in JavaScript, it won’t match the transformed ID.


Don’t use the window or document global properties to query for DOM elements. Also, we don’t recommend using JavaScript to manipulate the DOM. It’s better to use HTML directives to write declarative code.

Access Elements Passed Via Slots

A component doesn’t own DOM elements that are passed to it via slots. These DOM elements aren’t in the component’s shadow tree. To access DOM elements passed in via slots, call this.querySelector() and this.querySelectorAll(). Because the component doesn’t own these elements, you don’t use this.template.querySelector() or this.template.querySelectorAll().

This example shows how to get the DOM elements passed to a child component from the child’s context. Pass the selector name, such as an element, to this.querySelector() and this.querySelectorAll(). This example passes the span element.

Compose Components Using Slots Versus Data

When creating components that contain other components, consider the lifecycle and construction of the component hierarchy using the declarative (slots) or data-driven approach.

Compose Using Slots

This pattern is common for building components declaratively.

To support this pattern, the component author uses the slot element. Although convenient for the consumer, the component author must manage the lifecycle of the content passed through the slot element.

Use Custom Events to Notify the Parent About Child Availability

The parent component needs to know when a child component is available for communication. On the parent component, attach an event handler on the slot element or on a div element that contains the slot element.

Handle the event to notify the parent of the child component. A globally unique Id is required for the parent component to work with its child components.

To dispatch the event from the child component, use the connectedCallback() method.

Notify the Parent Component About Unregistered Child Component

To notify the parent that its child component is no longer available, we establish a two-way communication channel between the parent and child component.

  • Child sends a callback to the parent during registration.
  • Parent calls the child via a callback, passing another callback as an argument.
  • Child invokes the callback on the parent when being unregistered.


Since the component has been removed from the page, we can still invoke callbacks but we can’t send an event from disconnectedCallback().

The child sends a callback to the parent using an event handler onprivateitemregister.

Handle the event to notify the parent that the child is no longer available.

The child component invokes the callback on the parent when being unregistered.

The child component notifies the parent that it’s no longer available.

Pass Data to Child Components

Once the registration process is complete, we can communicate data between parent and child components via the exposed callback methods.

The parent component can pass data to a child component. For example, you can pass a string to a child component so that it can set a value for a setAriaLabelledBy attribute.

The child component sets the string on the attribute.

Compose Using Data

You observed that the declarative way to compose components adds a layer of complexity for the component author. Now consider the data-driven approach. Instead of managing the lifecycle for slot content and requiring granular management between parent and child components, the component gets the changes in a reactive way when data changes.

This example composes a child component using a data-driven approach.

To pass in data, use a JavaScript object. The child component reacts to data changes exclusively from its parent.

A data-driven approach is recommended when you have a complex use case.

Shadow DOM

The elements in each Lightning web component are encapsulated in a shadow tree. A shadow tree is part of the DOM that’s hidden from the document that contains it. The shadow tree affects how you work with the DOM, CSS, and events.

Shadow DOM is a web standard that encapsulates the elements of a component to keep styling and behavior consistent in any context. Since not all browsers implement Shadow DOM, Lightning Web Components uses a shadow DOM polyfill (@lwc/synthetic-shadow). A polyfill is code that allows a feature to work in a web browser.

To better understand how to work with the shadow tree, let’s look at some markup. This markup contains two Lightning web components: example-todo-app and example-todo-item. The shadow root defines the boundary between the DOM and the shadow tree. This boundary is called the shadow boundary.


The shadow root isn’t an element, it’s a document fragment.

Let’s look at how to work with the shadow tree in each of these areas.


CSS styles defined in a parent component don’t leak into a child. In our example, a p style defined in the todoApp.css style sheet doesn’t style the p element in the example-todo-item component, because the styles don’t reach into the shadow tree. See CSS.


If an event bubbles up and crosses the shadow boundary, to hide the internal details of the component that dispatched the event, some property values change to match the scope of the listener. See Event Retargeting.

Access Elements

To access elements a component renders from the component’s JavaScript class, use the template property. See Access Elements the Component Owns.

Access Slots

A slot is a placeholder for markup that a parent component passes into a component’s body. DOM elements that are passed to a component via slots aren’t owned by the component and aren’t in the component’s shadow tree. To access DOM elements passed in via slots, call this.querySelector() and this.querySelectorAll(). The component doesn’t own these elements, so you don’t use template. See Pass Markup into Slots.