Скачать или смотреть How to Use switchMap to Return Values from Earlier Observables in Angular/RxJS

Discover how to leverage `switchMap` in Angular/RxJS to return values from earlier observables, and efficiently combine data responses.
---
This video is based on the question https://stackoverflow.com/q/72795127/ asked by the user 'Brandon Graze' ( https://stackoverflow.com/u/18399226/ ) and on the answer https://stackoverflow.com/a/72797194/ provided by the user 'Eliseo' ( https://stackoverflow.com/u/8558186/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to have switchmap return values from earlier observable

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Use switchMap to Return Values from Earlier Observables in Angular/RxJS

When working with Angular and RxJS, one common challenge developers face is combining data from multiple observables. In this guide, we’ll tackle a specific problem: how to use the switchMap operator to return values from earlier observables by appending additional data based on a condition. Let's explore this concept using a practical example.

The Problem at Hand

Imagine you have an observable that retrieves customer information and you need to use a specific customer's number to look up their name. You want to merge this customer's name with the work item details you initially fetched, but you're unsure how to structure your RxJS code to achieve this.

Here's your initial observable setup:

[[See Video to Reveal this Text or Code Snippet]]

In this setup, you’re using the iif operator to determine whether a customer number exists, but you might be questioning if that’s the correct approach to take.

The Solution: Using switchMap

Instead of iif, we can utilize the switchMap operator which allows us to switch to a new observable based on the results of the original observable. This way, we can seamlessly handle the condition and return the necessary combined information.

Step-by-Step Implementation

Here’s a refined approach to solving your problem using switchMap:

Use switchMap for Conditional Logic: This operator helps in changing the observable to another observable based on the condition.

Transforming Data: If the customer number is available, we will fetch the customer's name and merge it with the existing work item details using the spread operator.

Here’s how to write the code:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Code

switchMap: We initiate a switch to a new observable based on the response (res) of the getWorkItem method.

customerNumber: We check if the customerNumber exists in the response.

getCustomerById: If the customer number is present, we call getCustomerById(res.customerNumber) to retrieve additional customer information.

map: We use the map operator to create a new object that comprises properties from both the original response (res) and the new customer data. Here, the spread operator { ...res, customerName: customer.name } is utilized to merge these objects efficiently.

of(res): If the customerNumber does not exist, we return the original response using the of operator.

Conclusion

By mastering the use of switchMap and understanding how to manipulate and combine observables, you can efficiently manage and enrich your Angular applications with richer data outputs. This approach not only enhances the readability of your code but also leverages powerful RxJS operators to handle asynchronous calls smoothly.

Don't hesitate to experiment with these techniques in your own projects, and stay tuned for more tips on working with Angular and RxJS!