LaunchDarkly feature flag service in Angular using RxJS

feature-flag.service.ts

import { Injectable, OnDestroy } from '@angular/core';
import { environment } from '@environments/environment';
import * as LaunchDarkly from 'launchdarkly-js-client-sdk';
import { LDFlagValue } from 'launchdarkly-js-client-sdk';
import { Observable, Subject } from 'rxjs';
import { map } from 'rxjs/operators';

@Injectable({
    providedIn: 'root'
})
export class FeatureFlagService implements OnDestroy {
    client: LaunchDarkly.LDClient;

    async ngOnDestroy() {
        await this.client.close();
    }

    initialize(): void {
        const user = {
            anonymous: true
        } as LaunchDarkly.LDUser;
        this.client = LaunchDarkly.initialize(environment.launchDarklyClientId, user);
    }

    getFlag(flagKey: string, defaultValue: LDFlagValue = false): Observable<LDFlagValue> {
        const fetchFlag = new Subject<void>();
        this.client.on(`change:${flagKey}`, () => {
            fetchFlag.next();
        });
        this.client.waitUntilReady().then(() => {
            fetchFlag.next();
        });
        return fetchFlag.pipe(
            map(() => {
                return this.client.variation(flagKey, defaultValue);
            })
        );
    }
}

How do you test “this.client.on” event? Any example code will be appreciated?
https://gist.github.com/cookavich/a8d99e8fa37b11ce8e0f3592d7e1e0c7#file-feature-flag-service-ts-L27