9. defer: a factory on every subscription

The problem: the value depends on when you subscribe

Picture this: you want an Observable that returns the current time. Write of(Date.now()) and the time "freezes" at the moment the Observable is created. Every subscriber gets the same value — even one that subscribes an hour later.

The fix: defer

defer(factory) takes a factory function. It's called afresh every time someone subscribes. Inside, you can compute an up-to-date value or build a brand-new Observable.

defer(() => of(Date.now()))
// Every subscriber gets a fresh Date.now()

Where it's useful

• The current date, or an up-to-date auth token.
• A fresh HTTP request: re-subscribing should fire a new request, not replay a cached one.
• Any value that should be computed at subscription time, not at creation time.

Glossary

• Factory — a function that builds something new on demand. In defer, it builds an Observable.

const { defer, of } = Rx;

let calls = 0;

const request$ = defer(() => {
  calls++;
  console.log('Factory call: ' + calls);
  return of('Data ' + calls);
});

console.log('Sub A');
request$.subscribe(value => console.log(value));

console.log('Sub B');
request$.subscribe(value => console.log(value));