Explample of async web service

webservice-async-example.ts

// Example of asynchronous web service
// Function that returns a promise with a list of contacts
// and throws if there is any error in Apex. As alternative,
// you can return empty array right there.
async getContacts(): Promise<Contact[]> {
  try {
    const contacts = await JSON.parse(sforce.apex.execute('AngularPOC', 'getContacts', { }));
    return contacts;
  } catch (e) {
    throw new Error(e);
  }
}

this.getContacts()
  .then(contacts => this.contacts = contacts)
  .catch(error => this.contacts = []);
// By the way, Angular has an async pipe so that you can
// display promises directly in the template.