Sample Code:
HTML:
<template>
<div class="slds-box slds-theme_default">
<lightning-accordion class="example-accordion"
onsectiontoggle={handleToggleSection}
allow-multiple-sections-open>
<lightning-accordion-section name="A" label="Accordion Title A">
Test
</lightning-accordion-section>
</lightning-accordion>
</div>
</template>
JavaScript:
import { LightningElement } from 'lwc';
export default class Example extends LightningElement {
handleToggleSection(event) {
console.log( 'Selected Sections ' + event.detail.openSections );
}
}
JavaScript meta.xml:
<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
<apiVersion>48.0</apiVersion>
<isExposed>true</isExposed>
<targets>
<target>lightning__Tab</target>
</targets>
</LightningComponentBundle>
Output:


