Retrieving Data using HTTP

Question 1

What is observable?

Answer 1

A collection of items over time.

Unlike an array, it doesn’t retain items. Emitted items can be observed over time.

Question 2

What does an observable do?

Answer 2

Nothing, until we subscribe. When we subscribe the observable emits notifications.
If everything is okay and item is emitted, next notification is emitted.
If an error occurs observable emits error notification. If there are no more items to emit, observable emits a complete notification.

Question 3

Common observable usage

Answer 3

1. Start the observable (subscribe)
2. Pipe emitted items through a set of operators
3. Process notifications: next. error, complete
4. Stop observable (unsubscribe)

Question 4

Variables that reference an observable

Answer 4

Should have ‘$’ as suffix

Question 5

HttpClientModule needs to be included

Answer 5

in the imports array of the app module

Question 6

Imports array of @NgModule is used for declaring

Answer 6

external modules

Question 7

Declarations array of @NgModule is used for

Answer 7

declaring components, directives and pipes that belong to that module

Question 8

HTTP get method returns an observable so

Answer 8

we will receive a notification. Which means
getProducts(): Observable {
return this.http.get(this.productUrl);
}

Question 9

To handle exceptions of observable we need

Answer 9

tap operator- taps intor observable stream without transforming the stream,
catchError -catches the error

Question 10

Subscribing to an observable

Answer 10

x.subscribe()
or
x.subscribe(Observer)
or
x.subscribe({ nextFn, erroFn, completeFn})

Subscribe returns subscription. We can use it later on to cancel subscription

Question 11

Unsubscribing from an Observable

Answer 11

It’s a good thing to always unsubscribe.

1. Store the subscription in a variable
2. Implement the OnDestroy lifecycle hook
3. Use the subscription variable to unsubscribe