Using concatMap in Angular

ConcatMap is an RxJS operator used in Angular to handle multiple requests sequentially. It is one of the most useful operators in RxJS, especially when dealing with asynchronous operations, and it can help developers write cleaner code with fewer bugs. In this article, we will explore when to use concatMap, how to use it, and its benefits.

What is concatMap?

ConcatMap is an RxJS operator that maps each value emitted by a source Observable to a new Observable, concatenates the resulting Observables, and returns this concatenated Observable. In other words, it will wait for the current Observable to complete before moving on to the next one.

ConcatMap is used when the order of the output should match the order of input of requests. For example, imagine an e-commerce website that needs to retrieve the user’s order history, payment information, and shipping details. The order history needs to be retrieved first, followed by payment information, and then shipping details. In this case, concatMap would be the perfect operator to use.

How to use concatMap in Angular?

To use concatMap in Angular, you first need to import it from the RxJS library:


import { concatMap } from 'rxjs/operators';

Then, you can use it in a pipe() function:


import { of } from 'rxjs';
import { concatMap } from 'rxjs/operators';

const source = of(1, 2, 3);
const example = source.pipe(
concatMap(val => of(`Delayed by: ${val}ms`).pipe(delay(val)))
);

example.subscribe(console.log);

The above code creates a source observable that emits the values 1, 2, and 3. These values are then passed through the concatMap operator. The concatMap operator uses the of() function to create a new observable that emits a string delayed by the value of the input. This new observable is then passed on to the next operator in the pipe() which is delay(). The delay() operator delays the emission of the value by the number of milliseconds passed as an argument. The delay() operator then emits the delayed string and passes it to the subscribe() function.

Benefits of using concatMap

There are several benefits of using concatMap in Angular:

  • Sequencing Requests: As mentioned earlier, concatMap is used to retrieve information sequentially, which means the requests will be triggered one after the other in a synchronous manner.

  • Reduced Complexity: ConcatMap reduces the complexity of the observable pipelines and makes them easier to understand and debug.

  • Improved Efficiency: ConcatMap can improve the efficiency of the code by reducing the number of unnecessary API calls.

FAQs

1. What is the difference between concatMap and mergeMap?

The main difference between concatMap and mergeMap is that concatMap subscribes to the source Observable and only moves on to the next Observable once the current one has completed. On the other hand, mergeMap subscribes to all of the Observables immediately and combines the outputs as quickly as possible.

2. When should I use concatMap instead of switchMap?

You should use concatMap when you need to preserve the order of the emitted results. SwitchMap cancels the previous inner Observable when a new one is received causing the order of the output to be different from the order of input of requests. In contrast, concatMap emits the results in the order that they were requested in the input, making it ideal for requests that need to be processed in sequence.

3. Can concatMap be used with observables created from event sources?

Yes, concatMap can be used with observables created from event sources such as clicks, scrolls, and mouse movements. However, it is important to note that concatMap should only be used if the order of the output needs to match the order of input of requests. If the order does not matter, then switchMap may be a better option as it can cancel the previous observable when a new one is received.

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