how to use the defer function from the rxjs library in javascript

defer() is a function provided by the rxjs library. It allows you to create an Observable that will only be created when an Observer subscribes to it. This is useful when you need to defer some expensive computation, such as making an HTTP request, until someone is actually interested in the result.

Here's an example of how to use defer():

index.tsx
const { defer } = require('rxjs');

// This is the expensive computation we want to defer
function getUserFromServer(id) {
  // ... some logic to fetch user from server ...
  return Promise.resolve({ id, name: 'Alice' });
}

// Observable that defers the getUserFromServer() function until subscribed to
const user$ = defer(() => getUserFromServer(123));

// Now we can subscribe to the user$ Observable
user$.subscribe(user => {
  console.log(user); // { id: 123, name: 'Alice' }
});
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In this example, we first create a getUserFromServer function that returns a Promise that resolves to a user object. Then we use defer() to create an Observable that will call getUserFromServer(123) only when someone subscribes to it. Finally, we subscribe to the resulting user$ Observable and log the user object to the console.

By using defer() we can ensure that the expensive getUserFromServer function is only called when needed, saving us resources and making our code more efficient.