Understanding the Purpose of the Pipe Function in Angular 2 and Typescript Observables

Question

Understanding the Purpose of the Pipe Function in Angular 2 and Typescript Observables

Recently, I encountered a situation where I needed to accept an Observer parameter in a function written in Typescript. I struggled to find a solution, reminding me of working with a delegate parameter in C#. The specific scenario involved adding a bookend for subscriptions to implement an AOP-like functionality. Although this detail is not crucial.

To make it functional, I had to modify the function parameter by appending a | character, resulting in:

(observer: Observer<any> | ((value: any) => void)): void

My question pertains to the purpose of the bolded section in the function signature. Despite extensive research, I have not been able to comprehend its significance. Initially, without the additional segment after the |, I received an error:

Argument of type '(res: any) => void' is not assignable to parameter of type 'Observer'. Property 'next' is missing in type '(res: any) => void'.

However, upon including the pipe character and the following segment, I could successfully subscribe in the traditional observer manner (using an arrow function). All subscriptions functioned as expected, eliminating the possibility of the segment after the pipe being a default value. Any insights on this would be greatly appreciated, as it is frustrating to witness something work effectively without understanding the underlying mechanism!

angular typescript angular2-observables

Answer 1

Answer №1

As mentioned by @echonax, it means OR as @Rob noted, it's a Union Type

Despite my slow pace, I decided to continue rather than discard what I've started :)

Initially, you limited your method to only accept Observers

class SubscribeMe<T> {
    observable: Observable<T>;
    susbscribe(observer: Observer<T>){
        return observable.susbscribe(observer);
    }
}

Then, you attempted to use an Action-like signature with it, where Action is defined as:

    type Action = <T>(t:T)=> void

By trying:

    susbscribeMe = new SubscribeMe<any>();
    susbscribeMe.susbscribe((x)=> console.log(x));

Unfortunately, this did not match the previous signature.

Subsequently, you added an Action-like signature to the method

 class SubscribeMe<T> {
    observable: Observable<T>
    susbscribe(observer: Observer<T>| (x)=> void ){
        return observable.susbscribe(observer);
    }
}

or...

 class SubscribeMe<T> {
    observable: Observable<T>
    susbscribe(observer: Observer<T>| Action<T>){
        return observable.susbscribe(observer);
    }
}

By doing this, you informed TypeScript that it can be called with an Action or an Observer

... and all of this is possible because the Rx.Observable interface (rxjs/Observable.d.ts)

has an overloaded signature for "subscribe" as follows:

 *  subscribe(): Subscription;
 *  subscribe(observer: PartialObserver<T>): Subscription;
 *  subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;

This means you can subscribe with:

an Action
an Observer
or Null/Nothing/Nada

but will always receive a Subscription in return

For example:

import * as Rx from "rxjs";

export class SubscribeMe<T> {

    private _events = new Rx.Subject<T>();
    
    /** 
    * Hide the source of the observable  
    * */
    get events(): Rx.Observable<T> { return this._events.asObservable(); }

    /**
    *  Accept any signature accepted by Rx.Observabe INTERFACE (rxjs/Observable.d.ts)
    *  subscribe(): Subscription;
    *  subscribe(observer: PartialObserver<T>): Subscription;
    *  subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;
    * 
    * as in OR Action<Ievent> Or Observer<Ievent> Or Null/Nothing/Nada
    */
    subscribe = (observer?: (e:T) => void | Rx.Observer<T>) => {
        return this.events.subscribe(observer);
    }
}

Answer 2

As mentioned by @echonax, it means OR as @Rob noted, it's a Union Type

Despite my slow pace, I decided to continue rather than discard what I've started :)

Initially, you limited your method to only accept Observers

class SubscribeMe<T> {
    observable: Observable<T>;
    susbscribe(observer: Observer<T>){
        return observable.susbscribe(observer);
    }
}

Then, you attempted to use an Action-like signature with it, where Action is defined as:

    type Action = <T>(t:T)=> void

By trying:

    susbscribeMe = new SubscribeMe<any>();
    susbscribeMe.susbscribe((x)=> console.log(x));

Unfortunately, this did not match the previous signature.

Subsequently, you added an Action-like signature to the method

 class SubscribeMe<T> {
    observable: Observable<T>
    susbscribe(observer: Observer<T>| (x)=> void ){
        return observable.susbscribe(observer);
    }
}

or...

 class SubscribeMe<T> {
    observable: Observable<T>
    susbscribe(observer: Observer<T>| Action<T>){
        return observable.susbscribe(observer);
    }
}

By doing this, you informed TypeScript that it can be called with an Action or an Observer

... and all of this is possible because the Rx.Observable interface (rxjs/Observable.d.ts)

has an overloaded signature for "subscribe" as follows:

 *  subscribe(): Subscription;
 *  subscribe(observer: PartialObserver<T>): Subscription;
 *  subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;

This means you can subscribe with:

an Action
an Observer
or Null/Nothing/Nada

but will always receive a Subscription in return

For example:

import * as Rx from "rxjs";

export class SubscribeMe<T> {

    private _events = new Rx.Subject<T>();
    
    /** 
    * Hide the source of the observable  
    * */
    get events(): Rx.Observable<T> { return this._events.asObservable(); }

    /**
    *  Accept any signature accepted by Rx.Observabe INTERFACE (rxjs/Observable.d.ts)
    *  subscribe(): Subscription;
    *  subscribe(observer: PartialObserver<T>): Subscription;
    *  subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): Subscription;
    * 
    * as in OR Action<Ievent> Or Observer<Ievent> Or Null/Nothing/Nada
    */
    subscribe = (observer?: (e:T) => void | Rx.Observer<T>) => {
        return this.events.subscribe(observer);
    }
}

Answer 3

Answer №2

When it says the observer is of type Observer<any>, it means that the observer is capable of handling any type of data.

Alternatively, the observer could also be a ((value: any) => void) function: This is a function that takes any type of value as input and returns nothing.

Answer 4

When it says the observer is of type Observer<any>, it means that the observer is capable of handling any type of data.

Alternatively, the observer could also be a ((value: any) => void) function: This is a function that takes any type of value as input and returns nothing.

Understanding the Purpose of the Pipe Function in Angular 2 and Typescript Observables

Answer №1

Answer №2

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