Modern Full-Stack Development

onDownloadProgress – This is a function to be called periodically 
while a response is downloading, allowing you the ability to show a 
progress bar or spinner or similar UI element (you can do this with 
onUploadProgress in the opposite direction too).
Axios is a very robust but extremely simple-to-use library that, for me, is the obvious 
choice for our server communication needs in MailBag.
 Mirroring the Server Part 1: Contacts.ts
The first Worker class we’re going to talk about is in the Contacts.ts file, which, of 
course, means we’re dealing with the Contacts Worker class. But, even before the Worker 
class, we find that we have an interface present:
export interface IContact {
  _id?: number, name: string, email: string
}
If you look back at the Contacts.ts file on the server side, you’ll find this same 
interface. That should make sense to you: after all, we’re passing objects back and forth 
that need to have the same structure on both sides of the conversation!
After that, the Worker class begins:
export class Worker {
Nothing special there, and again identical to the server. Within the Worker, we find a 
series of methods, beginning with listContacts().
Chapter 9   Delivering the gooDs: MailBag, the Client

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