lightning-accordion with Expand All and Collapse All in Salesforce Lightning Web Component(LWC)

Sample Code:
 
HTML:
<template>
    <lightning-card title="Accordion Example">
        <div class="slds-m-around_medium">
            <lightning-button
                label="Expand All"
                onclick={expandAll}
                class="slds-m-around_medium">
            </lightning-button>
            <lightning-button
                label="Collapse All"
                onclick={collapseAll}
                class="slds-m-around_medium">
            </lightning-button>
        </div>
        <lightning-accordion allow-multiple-sections-open class="slds-m-around_medium" active-section-name={activeSections}>
            <lightning-accordion-section name="A" label="Accordion Title A">
                <p>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing
                </p>
            </lightning-accordion-section>
            <lightning-accordion-section name="B" label="Accordion Title B">
                <p>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing
                </p>
            </lightning-accordion-section>
            <lightning-accordion-section name="C" label="Accordion Title C">
                <p>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing<br/>
                    Testing
                </p>
            </lightning-accordion-section>
        </lightning-accordion>
    </lightning-card>
</template>


JavaScript:
import { LightningElement } from 'lwc';

export default class Accordion extends LightningElement {

    activeSections = [ 'A', 'B', 'C' ];

    expandAll() {

        this.activeSections = [ 'A', 'B', 'C' ];

    }

    collapseAll() {

        this.activeSections = [];

    }

}


JS-meta.xml:
<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>52.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>

 
Output: