What is the purpose of the Tap operator in Rxjs?

Question

What is the purpose of the Tap operator in Rxjs?

Recently, I've started diving into the world of Rxjs and came across a tutorial by David Acosta on Rxjs operators. He mentioned that using the tap operator is useful when we don't need to alter the data of an observable. The data inside the tap function remains unchanged.

I decided to try out the following code:

 const source = Observable.of("david");

source.pipe(
  tap(x => x.toString().toUpperCase())
).subscribe(x => console.log(x));

The output I received was david. However, running this code:

source.subscribe(x => console.log(x));

also resulted in the same output david. This made me wonder why the tap operator is needed. The concept seemed like a mystery to me as I struggled to find proper resources explaining the TAP operator.

Can someone provide a detailed explanation on this and recommend some reliable tutorials or documentation for Rxjs operators?

typescript rxjs

Answer 1

Answer №1

The tap operator serves two main purposes:

1- Tracking the value of an observable stream:

const source = Observable.of("david");

let name;

source.pipe(
   tap(x => name = x)
).subscribe(x => console.log(x));

console.log('value stored in name: ', name);

2- Carrying out a side effect that needs to occur at a specific point in the pipeline (not during subscription):

clickStream$.pipe(
   tap((event) => {
       event.stopPropagation();
       event.preventDefault();
   }),
   debounce(300),
   map((event) => event.key)
).subscribe((key) => console.log('debounced key: ', key))

Important: Exercise caution when using the tap operator for side effects. While some are valid, others may be better handled within the .subscribe callback.

Trust this explanation helps!

Answer 2

The tap operator serves two main purposes:

1- Tracking the value of an observable stream:

const source = Observable.of("david");

let name;

source.pipe(
   tap(x => name = x)
).subscribe(x => console.log(x));

console.log('value stored in name: ', name);

2- Carrying out a side effect that needs to occur at a specific point in the pipeline (not during subscription):

clickStream$.pipe(
   tap((event) => {
       event.stopPropagation();
       event.preventDefault();
   }),
   debounce(300),
   map((event) => event.key)
).subscribe((key) => console.log('debounced key: ', key))

Important: Exercise caution when using the tap operator for side effects. While some are valid, others may be better handled within the .subscribe callback.

Trust this explanation helps!

Answer 3

Answer №2

Using the tap function differs from setting up a subscription in certain scenarios. Consider a situation where you have a connection to an HTTP server, the code might look like this:

function getItems(): Observable<Item[]> {
  return makeSomeExpensiveHttpCall()
    .map(rsp => doSomeExpensiveParsing(rsp))
    .tap(items => console.log(`Received ${items.length} items`));    
}

Initially, it appears similar to this version:

function getItems(): Observable<Item[]> {
  const result = return makeSomeExpensiveHttpCall()
    .map(rsp => doSomeExpensiveParsing(rsp));
  result.subscribe(items => {
    console.log(`Received ${items.length} items`);
  });
  return result;
}

However, the difference lies in the fact that each time you use subscribe, it triggers a new execution of both makeSomeExpensiveHttpCall and doSomeExpensiveParsing. This means in the second scenario, these functions are likely to be called twice instead of once, resulting in duplicate HTTP requests being sent to the server. This is something we want to avoid.

Answer 4

Using the tap function differs from setting up a subscription in certain scenarios. Consider a situation where you have a connection to an HTTP server, the code might look like this:

function getItems(): Observable<Item[]> {
  return makeSomeExpensiveHttpCall()
    .map(rsp => doSomeExpensiveParsing(rsp))
    .tap(items => console.log(`Received ${items.length} items`));    
}

Initially, it appears similar to this version:

function getItems(): Observable<Item[]> {
  const result = return makeSomeExpensiveHttpCall()
    .map(rsp => doSomeExpensiveParsing(rsp));
  result.subscribe(items => {
    console.log(`Received ${items.length} items`);
  });
  return result;
}

However, the difference lies in the fact that each time you use subscribe, it triggers a new execution of both makeSomeExpensiveHttpCall and doSomeExpensiveParsing. This means in the second scenario, these functions are likely to be called twice instead of once, resulting in duplicate HTTP requests being sent to the server. This is something we want to avoid.

Answer 5

Answer №3

For instance, the tap method can be utilized when you wish to perform a certain action every time you receive a value from your stream, but only want to retain values of a specific type by filtering out the rest.

Imagine you are constructing an array containing only numbers divisible by 2, excluding all other numbers.

Additionally, incorporating tap in your codebase often results in cleaner and more readable code.

Here is a simple code snippet for reference:

myObservable$ = new Observable(observer => {
    observer.next(1);
    observer.next(2);
    observer.next(3);
    observer.next(4);
    observer.next(5);
    observer.next(6);
    observer.complete();
});


this.myObservable$
  .pipe(
    tap(() => console.log('got Value!')),
    filter(value => (value % 2) === 0),
  )
  .subscribe(value => {
    console.log('filtered: ', value);
    this.mySpecificArray.push(value);
   });

Answer 6

For instance, the tap method can be utilized when you wish to perform a certain action every time you receive a value from your stream, but only want to retain values of a specific type by filtering out the rest.

Imagine you are constructing an array containing only numbers divisible by 2, excluding all other numbers.

Additionally, incorporating tap in your codebase often results in cleaner and more readable code.

Here is a simple code snippet for reference:

myObservable$ = new Observable(observer => {
    observer.next(1);
    observer.next(2);
    observer.next(3);
    observer.next(4);
    observer.next(5);
    observer.next(6);
    observer.complete();
});


this.myObservable$
  .pipe(
    tap(() => console.log('got Value!')),
    filter(value => (value % 2) === 0),
  )
  .subscribe(value => {
    console.log('filtered: ', value);
    this.mySpecificArray.push(value);
   });

Answer 7

Answer №4

When I think about it, it reminds me of the functionality of tee in Unix. Just like how you can use tee to split a stream and write it to a file while still processing it further down the pipeline, for example:

echo "d" | tee output.txt | sed 's/d/e/g'
# console: e
# output.txt: d

In this scenario, the data "d" is sent through tee. Tee then copies the stream to a file without affecting the original stream, allowing it to proceed unchanged to the next command, which here is sed.

Answer 8

When I think about it, it reminds me of the functionality of tee in Unix. Just like how you can use tee to split a stream and write it to a file while still processing it further down the pipeline, for example:

echo "d" | tee output.txt | sed 's/d/e/g'
# console: e
# output.txt: d

In this scenario, the data "d" is sent through tee. Tee then copies the stream to a file without affecting the original stream, allowing it to proceed unchanged to the next command, which here is sed.

What is the purpose of the Tap operator in Rxjs?

Answer №1

Answer №2

Answer №3

Answer №4

Similar questions

What is the reason for TypeScript allowing this promise chain without any compilation errors?

`Finding and accessing the attributes and object of a React child component from its parent component`

Enhance existing functionalities in library with type augmentation strategy

How can I prevent node_module from being included when using the include directive in tsconfig.json?

Issue with Typescript and rxjs 6: Property is not found on type 'object'

Guide to creating numerous separate subscriptions in angular 6

The object does not contain a 'navigation' property within the 'Readonly<{}> & Readonly<{ children?: ReactNode; }>' type

Leveraging NgRx effects with mergeMap

The canActivate() function encounters issues when working with Observable responses in Angular 2

Pause and be patient while in the function that delivers an observable

Property missing in Typescript type definition

Incorporate a fresh element into an object after its initial creation

Tips for showcasing information of a particular child in a database

What is the reason for the function to return 'undefined' when the variable already holds the accurate result?

How Can I Build a Dynamic Field Form Builder in Angular 4?

What is the process for managing multiple selections using checkbox code?

The system does not acknowledge 'NODE_OPTIONS' as a command that can be used internally or externally, or as an operational program or batch file

Retrieving the chosen option from a personalized drop-down element

The presence of an Angular pipe is causing interference with the execution of a template

How can you define types for abstract React components in Typescript?