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RxJS from the ground up - Introduction to reactive JavaScript

Yannick Baron
February 18, 2021

RxJS from the ground up - Introduction to reactive JavaScript

RxJS is a very powerful tool and has been established in Angular as its central way of communication between Components and Services.

If you are not quite comfortable yet to use RxJS in all of its glory, then this Webinar is for you. Yannick is going to show you the basic concepts and ideas behind all of RxJS's moving parts, to make sure you can wield this powerful tool more confidently.

- Why streams?
- Asynchronicity and the observer pattern
- Working with RxJS
- Unsubscribing and completion
- Resources to help yourself

Yannick Baron

February 18, 2021

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  1. RxJS from the ground up

    Introduction to reactive JavaScript
    Yannick Baron

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  2. Intro: Why Streams?

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  3. • various forms of asynchronicity in modern web applications

    • WebSockets

    • DOM Events

    • AJAX
    Dealing with Asynchronicity

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  4. • designed to handle asynchronicity

    • start running the moment you create it

    • will always resolve or reject

    • handles a single event/value

    → can pretty much only cover AJAX

    • not cancellable

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  5. • software design pattern

    • a Subject noti
    es a list of Observers by calling a provided method

    • Observers register and unregister with the Subject

    • useful to be noti
    ed of multiple events or actions
    The observer pattern

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  6. • programming paradigm

    • reacting to asynchronous events

    (e.g. user input, data updates, asynchronous responses)

    • data streams and propagation of change
    Reactive programming

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  7. • library implementing the Observer pattern

    • API to describe data streams

    • shines when handling multiple reactive events

    • lots of useful operators

    retrying, error handling, delaying, timing,

    → for more than just HTTP requests
    Rx - Reactive Extensions

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  8. • designed to handle asynchronicity

    • will generally not run until there is a subscriber

    • handles sequence of 0..N asynchronous events over time

    → can handle multiple forms of asynchronicity

    • can be unsubscribed from (can notify producer)

    • communicate change or updates to subscribers

    • combine multiple streams and de
    ne how data

    • many operators to help control the
    Rx - On Observables and Streams

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  9. Practical Introduction

    working with RxJS

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  10. • Observable

    data stream or source emitting data over time

    • Observer

    function that will be called when new data is emitted

    • Subscription

    one-sided connection between Observer and Observable

    • Operators (combination,
    ltering, transformation, ...)

    replace your observables with a modi
    ed one
    Observable, Observer, Subscription & Operators

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  11. new Observable(observer => {

    observer.next('Value 1');

    observer.next('Value 2');


    return () => {

    // observer unsubscribed



    from(['Value 1', 'Value 2']);

    of('Value 1');

    create, fromEvent, interval, ...

    creating Observables is easy!

    wrap asynchronous actions

    several (12) creation operators

    [Examples: 01]

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  12. • three events we can react to

    • next

    receive the next value going through the stream

    • error

    react to the observable throwing - no more values will be emitted

    observers unsubscribe - observable does not complete

    • complete

    observable completes without error - no more values will be emitted

    observers unsubscribe

    next, error, complete
    [Examples: 02, 03]

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  13. • Observable

    data stream or source emitting data over time

    is cold / unicast: start upon subscription with new execution context

    → single listener (subscriber)

    • Subject (AsyncSubject / BehaviorSubject / ReplaySubject)

    is an Observable that we can tell when to next / error / complete

    hot / multicast: run without subscription and keep execution context

    → multiple listeners (subscriber)
    Subject & Observable - Hot & Cold
    [Examples: 04, 05]

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  14. • Subject

    emits values regularly - you can miss values

    • AsyncSubject

    only emits the last value once the subject completes

    • BehaviorSubject (stateful)

    emits default value or last emitted value upon subscription, then regularly

    • ReplaySubject

    emits the last N values upon subscription, then regularly
    [Examples: 05, 06, 07, 08]

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  15. • Operators (combination,
    ltering, transformation, ...)

    replace your observables with a modi
    ed one

    • https://rxmarbles.com/
    Operators and marble diagrams
    Events over time
    Resulting Observable

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  16. Unsubscribing and Completion
    Always unsubscribe?

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  17. • noti
    es Subject to remove Observer

    • Observable can perform cleanup logic

    e.g. cancelling an http request, unsetting timers, closing connections, ...

    • will stop Subscription from executing

    if subscription is kept alive on a destroyed Component, it will still execute

    can cause memory leaks

    • possibility to unsubscribe via operators

    • Subject completion causes the Observers to unsubscribe
    [Examples: leak]

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  18. • No... Well... Not directly...

    • ... if you know exactly what you are doing.

    • Always unsubscribing is the safest way to prevent leaks or side-effects

    rst step

    more comfortable with RxJS → easily identify when to unsubscribe

    • Unsubscribe cleverly. At the right moment. In the most elegant way.

    reason about it - make your app behave the way you need it to

    make use of operators

    (In Angular: async pipe unsubscribes for you!)

    Do we always have to unsubscribe?

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  19. Getting Help
    learning to help yourself

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  20. • https://www.learnrxjs.io/

    explains all operators with examples and more

    • https://rxmarbles.com/

    visual representation of some operators

    • https://stackblitz.com/

    quickly construct a demo for your use case

    • https://rxjs-dev.

    helps you
    nd the right operator, good in the beginning
    Great resources

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  21. It's a wrap!

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