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Events with data demonstrate how to pass information from a child component to a parent component using the detail property of CustomEvent.

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Overview

This example shows a contact list where clicking a contact item sends the contact ID from the child component to the parent, which then displays the full contact details.

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Child Component: c-contact-list-item

The contact list item component dispatches an event containing the contact ID.

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JavaScript (contactListItem.js)

import { LightningElement, api } from 'lwc';

export default class ContactListItem extends LightningElement {
    @api contact;

    handleClick(event) {
        // 1. Prevent default behavior of anchor tag click
        event.preventDefault();
        
        // 2. Create custom event with data in the detail property
        const selectEvent = new CustomEvent('select', {
            detail: this.contact.Id
        });
        
        // 3. Fire the custom event
        this.dispatchEvent(selectEvent);
    }
}
Key Points:
  • The detail property carries data from child to parent
  • detail can contain any serializable data (primitives, objects, arrays)
  • Receives contact data via @api contact property

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Template (contactListItem.html)

<template>
    <a href="#" onclick={handleClick}>
        <lightning-layout vertical-align="center">
            <lightning-layout-item>
                <img src={contact.Picture__c} alt="Profile photo" />
            </lightning-layout-item>
            <lightning-layout-item padding="around-small">
                <p>{contact.Name}</p>
            </lightning-layout-item>
        </lightning-layout>
    </a>
</template>

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Parent Component: c-event-with-data

The parent component handles the event and extracts data from event.detail.

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JavaScript (eventWithData.js)

import { LightningElement, wire } from 'lwc';
import getContactList from '@salesforce/apex/ContactController.getContactList';

export default class EventWithData extends LightningElement {
    selectedContact;

    @wire(getContactList) contacts;

    handleSelect(event) {
        // Extract contact ID from event.detail
        const contactId = event.detail;
        
        // Find and store the selected contact
        this.selectedContact = this.contacts.data.find(
            (contact) => contact.Id === contactId
        );
    }
}
Key Points:
  • Access event data via event.detail
  • The detail value is exactly what the child component provided
  • Use the data to update parent component state

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Template (eventWithData.html)

<template>
    <lightning-card title="EventWithData" icon-name="standard:logging">
        <template lwc:if={contacts.data}>
            <lightning-layout class="slds-var-m-around_medium">
                <lightning-layout-item>
                    <!-- Each element must have a listener for non-bubbling events -->
                    <template for:each={contacts.data} for:item="contact">
                        <fieldset key={contact.Id} class="slds-var-p-horizontal_x-small">
                            <legend>c-contact-list-item</legend>
                            <c-contact-list-item
                                class="slds-show slds-is-relative"
                                contact={contact}
                                onselect={handleSelect}
                            ></c-contact-list-item>
                        </fieldset>
                    </template>
                </lightning-layout-item>
                <lightning-layout-item class="slds-var-m-left_medium">
                    <template lwc:if={selectedContact}>
                        <img src={selectedContact.Picture__c} alt="Profile photo" />
                        <p>{selectedContact.Name}</p>
                        <p>{selectedContact.Title}</p>
                        <p>
                            <lightning-formatted-phone
                                value={selectedContact.Phone}
                            ></lightning-formatted-phone>
                        </p>
                        <p>
                            <lightning-formatted-email
                                value={selectedContact.Email}
                            ></lightning-formatted-email>
                        </p>
                    </template>
                </lightning-layout-item>
            </lightning-layout>
        </template>
        <template lwc:elseif={contacts.error}>
            <c-error-panel errors={contacts.error}></c-error-panel>
        </template>
    </lightning-card>
</template>
Key Points:
  • Non-bubbling events require listeners on each child component
  • Event handler onselect binds to the ‘select’ event
  • Parent displays selected contact details when a contact is chosen

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CustomEvent Options

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detail

Carries data from child to parent. Can be any type: 
// String
new CustomEvent('select', { detail: 'some-id' })

// Number
new CustomEvent('select', { detail: 42 })

// Object
new CustomEvent('select', { detail: { id: '123', name: 'John' } })

// Array
new CustomEvent('select', { detail: [1, 2, 3] })

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Default Behavior

By default, custom events:
  • Do NOT bubble up the DOM tree (bubbles: false)
  • Do NOT cross shadow DOM boundaries (composed: false)

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Event Flow

  1. User clicks a contact item
  2. Child component creates CustomEvent with contact ID in detail
  3. Child dispatches the event
  4. Parent’s onselect handler receives the event
  5. Parent extracts event.detail to get the contact ID
  6. Parent finds and displays the full contact information

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Source Files

  • Child: force-app/main/default/lwc/contactListItem/
  • Parent: force-app/main/default/lwc/eventWithData/

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