Get Selected Rows In Lightning Datatable In LWC

public with sharing class DataController {
    @AuraEnabled (cacheable=true)
    public static List<Account> retrieveAccounts(){
        return [SELECT Id, Name, Type, BillingCountry
                FROM Account
                LIMIT 2000];
    }
}

<template>
    <lightning-Card title="Account List" icon-name="standard:account">
        <lightning-button   variant="Neutral" 
                            label="Selected Records"
                            title="Selected Records"
                            onclick={getSelectedRec} 
                            slot="actions"
                            icon-name="utility:check">
        </lightning-button>
        <div style="border-top: 1px solid rgb(221 219 218);">
            <!-- datatable -->
            <lightning-datatable
                    key-field="id"
                    data={data}
                    columns={columns}
                    column-widths-mode="fixed">
            </lightning-datatable>
            <!-- /datatable -->
        </div>
    </lightning-Card>
</template>

import { LightningElement, track, wire } from 'lwc';
import retrieveAccounts from '@salesforce/apex/DataController.retrieveAccounts';
const columns = [
    { label: 'Name', fieldName: 'Name' },
    { label: 'Type', fieldName: 'Type' },
    { label: 'BillingCountry', fieldName: 'BillingCountry'},       
];

export default class SelectedRowsLWC extends LightningElement {
    @track data;
    @track columns = columns;
    @track error;

    @wire(retrieveAccounts)
    wiredAccounts({ error, data }) {
        if (data) {
            this.data = data;
            this.error = undefined;
        } else if (error) {
            this.error = error;
            this.data = undefined;
        }
    }

    getSelectedRec() {
      var selectedRecords =  this.template.querySelector("lightning-datatable").getSelectedRows();
      if(selectedRecords.length > 0){
          console.log('selectedRecords are ', selectedRecords);

          let ids = '';
          selectedRecords.forEach(currentItem => {
              ids = ids + ',' + currentItem.Id;
          });
          this.selectedIds = ids.replace(/^,/, '');
          this.lstSelectedRecords = selectedRecords;
          alert(this.selectedIds);
      }   
    }
}