Let's have a look at the marble diagram for this operator: Going back to our previous form draft save example, its clear that what we need concatMap in that case and not mergeMap, because we don't want the saves to happen in parallel. Choosing the wrong operator often does not result in an immediatelly broken program, but it might lead to some hard to troubleshoot issues over time. So without further ado, let's get started with our RxJs mapping operators deep dive! Map map is the most common operator in Observables. In both examples we use the Observable map operator to modify the data that is being emitted by the Observable. map, mergeMap and switchMap are three principal operators in RxJS that you would end up using quite often. * switchMap which would switch to a new inner observable each emission, exhaustMap for login effect in @ngrx example app ( Source) @ Effect Concatmap . For example, most of the network calls in our program are going to be done using one of these operators, so getting familiar with them is essential in order to write almost any reactive program. These operators might seem unrelated, but we really want to learn them all in one go, as choosing the wrong operator might accidentally lead to subtle issues in our programs. Let’s look at some examples: We first created our Observable with an array of cars. Photo by Geran de Klerk on Unsplash. It merges the values from all of its inner observables and emits the values … If you want to try out the examples in this post, here is a playground repository containing the running code for this post. This way the data gets progressively saved as the user fills in the form, which avoids losing the whole form data due to an accidental reload. The result is a higher-order Observable. In order to implement sequential saves, we are going to introduce the new notion of Observable concatenation. Other RxJS flattening operators include mergeMap (aka FlatMap) and concatMap. switchMap, as well as other **Map operators, will substitute value on the source stream with a stream of values, returned by inner function. It creates a new inner observable for every value it receives from the Source. Let's see what it would take to ensure that a save request is done only after the previous save is completed. But first things first. This operator is generally considered a safer default to mergeMap! should we cancel an ongoing save and start a new one? Este curso de ReactiveX está orientado a enseñarte desde los fundamentos de las extensiones reactivas hasta los detalles del tema.Pueda que ya usemos las extensiones reactivas en nuestras aplicaciones, ya que muchos frameworks lo traen incluido, pero usualmente no lo aprovechamos al máximo por desconocimiento del tema. I hope that you have enjoyed this post! March 13, 2018 • 3 minute read. What the doc says: Maps each value to an Observable, then flattens all of these inner Observables using mergeAll. concatMap is taking each form value and transforming it into a save HTTP Observable, called an, concatMap then subscribes to the inner Observable and sends its output to the result Observable, a second form value might come faster than what it takes to save the previous form value in the backend, If that happens, that new form value will, instead, concatMap will wait for previous HTTP Observable to complete, each value of the source Observable is still being mapped into an inner Observable, just like the case of concatMap, Like concatMap, that inner Observable is also subscribed to by mergeMap, as the inner Observables emit new values, they are immediately reflected in the output Observable, but unlike concatMap, in the case of mergeMap we don't have to wait for the previous inner Observable to complete before triggering the next innner Observable, this means that with mergeMap (unlike concatMap) we can have multiple inner Observables overlapping over time, emitting values in parallel like we see highlighted in red in the picture, the higher-order Observable emits its first inner Observable (a-b-c-d), that gets subscribed to (by the switch strategy implementation), the first inner Observable (a-b-c-d) emits values a and b, that get immediately reflected in the output, but then the second inner Observable (e-f-g) gets emitted, which, the second inner Observable (e-f-g) then starts emitting new values, that get reflected in the output, but notice that the first inner Observable (a-b-c-d) is meanwhile, the source observable emits values 1, 3 and 5, these values are then turned into Observables by applying a mapping function, the mapped inner Observables get subscribed to by switchMap, when the inner Observables emit a value, the value gets immediately reflected in the output, notice the 30-30-30 inner Observable in red in the diagram above: the last 30 value was not emitted because the 30-30-30 inner Observable got unsubscribed from, Just like in the case of switch, exhaust is subscribing to the first inner Observable (a-b-c), The value a, b and c get immediately reflected in the output, as usual, then a second inner Observable (d-e-f) is emitted, while the first Observable (a-b-c) is still ongoing, only after the first Observable (a-b-c) completes, will the exhaust strategy subscribe to new Observables, when the third Observable (g-h-i) is emitted, the first Observable (a-b-c) has already completed, and so this third Observable will not be discarded and will be subscribed to, the values g-h-i of the third Observable will then show up in the output of the result Observable, unlike to values d-e-f that are, the value 1 gets emitted, and a inner Observable 10-10-10 is created, the Observable 10-10-10 emits all values and completes before the value 3 gets emitted in the source Observable, so all 10-10-10 values where emitted in the output, a new value 3 gets emitted in the input, that triggers a new 30-30-30 inner Observable, but now, while 30-30-30 is still running, we get a new value 5 emitted in the source Observable, this value 5 is discarded by the exhaust strategy, meaning that a 50-50-50 Observable was never created, and so the 50-50-50 values never showed up in the output. 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