What advantages does subscribe() offer compared to map() in Angular?

Question

What advantages does subscribe() offer compared to map() in Angular?

I recently started exploring observables in angular2 and found myself puzzled over the decision to use map() instead of subscribe(). Let's say I am fetching data from a webApi, like so:

  this.http.get('http://172.17.40.41:8089/api/Master/GetAllCountry')

With

subscribe(success, error, complete)

, I can retrieve all values in the success callback and return the values in the complete callback. So, why would I need to use map()? Does it offer any advantages?

In essence, why should one write code like this:

this.http.get('http://172.17.40.41:8089/api/Master/GetAllCountry')
    .map(r=>{})
    .subscribe(value => {
    }, error => error, () => {
});

when they could simply write this without using the map function:

this.http.get('http://172.17.40.41:8089/api/Master/GetAllCountry')
    .subscribe(value => {        
    }, error => error, () => {           
});

angular typescript rxjs observable angular-observable

Answer 1

Answer №1

When you need to pass on an Observable for other code to subscribe to, but also want to manipulate the data events within your current method, then you should utilize the map function.

Keep in mind that the user of the observable must call subscribe(), as without it the observable will not execute at all. Alternatively, using methods like forEach() or toArray() (and others) can serve the same purpose of executing the observable instead of directly calling subscribe().

The result of calling subscribe() is a Subscription object which cannot be subscribed to itself, though it can be used to cancel the subscription when needed.

On the other hand, invoking map() will return a new Observable that can indeed be subscribed to by other parts of the code.

Answer 2

When you need to pass on an Observable for other code to subscribe to, but also want to manipulate the data events within your current method, then you should utilize the map function.

Keep in mind that the user of the observable must call subscribe(), as without it the observable will not execute at all. Alternatively, using methods like forEach() or toArray() (and others) can serve the same purpose of executing the observable instead of directly calling subscribe().

The result of calling subscribe() is a Subscription object which cannot be subscribed to itself, though it can be used to cancel the subscription when needed.

On the other hand, invoking map() will return a new Observable that can indeed be subscribed to by other parts of the code.

Answer 3

Answer №2

Consider the map function as a middleware that modifies the response.

this.http.get('http://172.17.40.41:8089/api/Master/GetAllCountry')
.map(response=>response.json())
 .subscribe(data => {
              // The data variable now holds the parsed JSON object after going through the map function...
            },onErrorCallback, onCompleteCallback)

The subscribe function is used to trigger the observable, it's recommended to learn more about cold-vs-hot-observables

Answer 4

Consider the map function as a middleware that modifies the response.

this.http.get('http://172.17.40.41:8089/api/Master/GetAllCountry')
.map(response=>response.json())
 .subscribe(data => {
              // The data variable now holds the parsed JSON object after going through the map function...
            },onErrorCallback, onCompleteCallback)

The subscribe function is used to trigger the observable, it's recommended to learn more about cold-vs-hot-observables

Answer 5

Answer №3

In order to execute your asynchronous request, it is essential to first subscribe. Simply setting the map function will not initiate any requests. You can verify this by testing.

It is highly recommended to utilize the map function for data preprocessing, as multiple subscribers may need access to your results. By preparing a single output with a unified data schema, you eliminate the need for individual preprocessing for each client (subscriber).

Answer 6

In order to execute your asynchronous request, it is essential to first subscribe. Simply setting the map function will not initiate any requests. You can verify this by testing.

It is highly recommended to utilize the map function for data preprocessing, as multiple subscribers may need access to your results. By preparing a single output with a unified data schema, you eliminate the need for individual preprocessing for each client (subscriber).

Answer 7

Answer №4

Observables are like flowing streams that are meant to be handled in a functional way. It is recommended to utilize RxJS operations as they embody the essence of functional programming when working with subscriptions and observables, especially when extracting data from the stream.

This scenario involves an asynchronous operation being executed synchronously:

bad_example() {
  let id;
  this.obs.subscribe(param => id = param['id']);
    this.get(id).subscribe(elem => this.elem = elem);
}

In contrast, this example demonstrates how an asynchronous operation should be appropriately handled as a stream:

good_example() {
  this.obs
    .map(param => param['id'])
    .switchMap(id => return this.get(id))
    .subscribe(elem => this.elem = elem);
}

Answer 8

Observables are like flowing streams that are meant to be handled in a functional way. It is recommended to utilize RxJS operations as they embody the essence of functional programming when working with subscriptions and observables, especially when extracting data from the stream.

This scenario involves an asynchronous operation being executed synchronously:

bad_example() {
  let id;
  this.obs.subscribe(param => id = param['id']);
    this.get(id).subscribe(elem => this.elem = elem);
}

In contrast, this example demonstrates how an asynchronous operation should be appropriately handled as a stream:

good_example() {
  this.obs
    .map(param => param['id'])
    .switchMap(id => return this.get(id))
    .subscribe(elem => this.elem = elem);
}

Answer 9

Answer №5

_forEach() is an important method in JavaScript, it enables you to iterate over each element of an array and perform a certain task on them.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

Answer 10

_forEach() is an important method in JavaScript, it enables you to iterate over each element of an array and perform a certain task on them.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach

What advantages does subscribe() offer compared to map() in Angular?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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