SYS-CON MEDIA Authors: Liz McMillan, Elizabeth White, Pat Romanski, Jason Bloomberg, Dana Gardner

Blog Feed Post

RxJS Essentials. Part 6: The switchMap operator

In this article I’ll introduce the switchMap() operator. The previous articles in this series include:

1. Basic Terms
2. Operators map, filter, and reduce
3. Using Observable.create()
4. Using RxJS Subject
5. The flatMap operator

While flatMap() unwraps and merges all the data from the outer observable values, the switchMap() operator handles the data from the outer observable but cancels the inner subscription being processed if the outer observable emits a new value. The switchMap() operator is easier to explain with the help of its marble diagram shown next.

https://yakovfain.files.wordpress.com/2017/09/appd_switchmap.png?w=1520&... 1520w, https://yakovfain.files.wordpress.com/2017/09/appd_switchmap.png?w=150&h=81 150w, https://yakovfain.files.wordpress.com/2017/09/appd_switchmap.png?w=300&h... 300w, https://yakovfain.files.wordpress.com/2017/09/appd_switchmap.png?w=768&h... 768w, https://yakovfain.files.wordpress.com/2017/09/appd_switchmap.png?w=1024&... 1024w" sizes="(max-width: 760px) 100vw, 760px" />

The outer observable emits the red circle, and switchMap() emits the item from the inner observable (red diamond and square) into the output stream. The red circle was processed without any interruptions because the green circle was emitted after the inner observable finished processing.

The situation is different with the green circle. The switchMap() managed to unwrap and emit the green diamond, but the blue circle arrived before the green square was processed. So the subscription to the green inner observable was cancelled, and the green square was never emitted into the output stream. In other words, the switchMap() operator switched from processing of the green inner observable to the blue one.

The following example has two observables. The outer observable uses the function interval() and emits a sequential number every second. With the help of the take() operator we limit the emission to two values: 0 and 1. Each of these values is given to the switchMap() operator, and the inner observable emits three numbers with the interval of 400 milliseconds.

let outer$ = Rx.Observable
               .interval(1000)
               .take(2);

let combined$ = outer$.switchMap((x) => {  
     return Rx.Observable
	          .interval(400)
	          .take(3)
	          .map(y => `outer ${x}: inner ${y}`)
});

combined$.subscribe(result => console.log(`${result}`));

The output of this script is shown next:

outer 0: inner 0
outer 0: inner 1
outer 1: inner 0
outer 1: inner 1
outer 1: inner 2

Note that the first inner observable didn’t emit its third value 2. Here’s the timeline:

1. The outer observable emits 0 and the inner emits 0 400 milliseconds later
2. In 800 milliseconds later, the inner observable emits 1
3. In 1000 milliseconds the outer observable emits 1, and inner observable was unsubscribed
4. The three inner emissions for the second outer value went uninterrupted because it didn’t emit any new values

If you replace flatMap() with switchMap() the inner observable will emit three values for each outer value as shown below.

outer 0: inner 0
outer 0: inner 1
outer 0: inner 2
outer 1: inner 0
outer 1: inner 1
outer 1: inner 2

NOTE: To see it in CodePen, follow this link.

The chances are slim that you’ll be writing outer and inner observables emitting integers. There are various practical use cases for switchMap() in the real world projects. For example, in my Angular apps (Angular comes with RxJS) I use switchMap() with the HttpClient object to cancel pending HTTP requests. Just think of a user that types in an HTML input field (the outer observable) and the HTTP requests are being made (inner observable) on each keyup event. The circles on the diagram are the three characters that the user is typing. The inner observables are HTTP requests issued for each character. If the user entered the third character but the HTTP request for the second one is still pending, it’ll get cancelled.

TIP. The function interval() is handy if you want to invoke another function periodically based on the specified time interval. For example, myObservable.interval(1000).subscribe(n => doSometing()) will result in calling the function doSomething() every second.

If you have an account at O’Reilly’s safaribooksonline.com, you can watch my video course “RxJS Essentials” there.


Read the original blog entry...

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Latest Stories
Eric Taylor, a former hacker, reveals what he's learned about cybersecurity. Taylor's life as a hacker began when he was just 12 years old and playing video games at home. Russian hackers are notorious for their hacking skills, but one American says he hacked a Russian cyber gang at just 15 years old. The government eventually caught up with Taylor and he pleaded guilty to posting the personal information on the internet, among other charges. Eric Taylor, who went by the nickname Cosmo...
René Bostic is the Technical VP of the IBM Cloud Unit in North America. Enjoying her career with IBM during the modern millennial technological era, she is an expert in cloud computing, DevOps and emerging cloud technologies such as Blockchain. Her strengths and core competencies include a proven record of accomplishments in consensus building at all levels to assess, plan, and implement enterprise and cloud computing solutions. René is a member of the Society of Women Engineers (SWE) and a m...
SUSE is a German-based, multinational, open-source software company that develops and sells Linux products to business customers. Founded in 1992, it was the first company to market Linux for the enterprise. Founded in 1992, SUSE is the world’s first provider of an Enterprise Linux distribution. Today, thousands of businesses worldwide rely on SUSE for their mission-critical computing and IT management needs.
Mid-sized companies will be pleased with StorageCraft's low cost for this solution compared to others in the market. There are no startup fees, our solution has a predictable monthly cost, highly competitive pricing and offers ongoing value for our partners month after month. By enabling pooling and StorageCraft's 30-days of free virtualization the company removes several concerns surrounding machine size management and disaster recovery testing costs that add to the complexity of implementing a...
Cryptomage, Inc. is a dynamic hi-tech ICT company offering products and services in the cybersecurity area. We deliver high quality solutions both technologically and conceptually. We have academic roots and hire scientists, cybersecurity experts, software developers and analysts. Our strategy is focused on maintaining the highest skills of our human resources. The Cryptomage team is capable of more than delivering complex solutions; we can improve the whole concept to deliver real added value t...
There's no doubt that blockchain technology is a powerful tool for the enterprise, but bringing it mainstream has not been without challenges. As VP of Technology at 8base, Andrei is working to make developing a blockchain application accessible to anyone. With better tools, entrepreneurs and developers can work together to quickly and effectively launch applications that integrate smart contracts and blockchain technology. This will ultimately accelerate blockchain adoption on a global scale.
In addition to 22 Keynotes and General Sessions, attend all FinTechEXPO Blockchain "education sessions" plus 40 in two tracks: (1) Enterprise Cloud (2) Digital Transformation. PRICE EXPIRES AUGUST 31, 2018. Ticket prices: ($295-Aug 31) ($395-Oct 31) ($495-Nov 12) ($995-Walk-in) Does NOT include lunch.
Early Bird Registration Discount Expires on August 31, 2018 Conference Registration Link ▸ HERE. Pick from all 200 sessions in all 10 tracks, plus 22 Keynotes & General Sessions! Lunch is served two days. EXPIRES AUGUST 31, 2018. Ticket prices: ($1,295-Aug 31) ($1,495-Oct 31) ($1,995-Nov 12) ($2,500-Walk-in)
DevOpsSUMMIT at CloudEXPO will expand the DevOps community, enable a wide sharing of knowledge, and educate delegates and technology providers alike. Recent research has shown that DevOps dramatically reduces development time, the amount of enterprise IT professionals put out fires, and support time generally. Time spent on infrastructure development is significantly increased, and DevOps practitioners report more software releases and higher quality. Sponsors of DevOpsSUMMIT at CloudEXPO will b...
Yottabyte is a software-defined data center (SDDC) company headquartered in Bloomfield Township, Oakland County, Michigan. The mission of Yottabyte is to enable any organization to easily and economically realize the benefits of the cloud in their own datacenter. The Yottabyte Cloud Software natively virtualizes compute, storage, and network resources into a resilient, automated software-defined cloud infrastructure solution. Both Enterprise IT customers and Service Providers benefit from Yottab...
FinTech Is Now Part of the CloudEXPO New York Program. Financial enterprises in New York City, London, Singapore, and other world financial capitals are embracing a new generation of smart, automated FinTech that eliminates many cumbersome, slow, and expensive intermediate processes from their businesses. Accordingly, attendees at the upcoming 22nd CloudEXPO | DXWorldEXPO November 12-13, 2018 in New York City will find fresh new content in two new tracks called: FinTechEXPO New York Blockchain E...
Only Adobe gives everyone - from emerging artists to global brands - everything they need to design and deliver exceptional digital experiences. Adobe Systems Incorporated develops, markets, and supports computer software products and technologies. The Company's products allow users to express and use information across all print and electronic media. The Company's Digital Media segment provides tools and solutions that enable individuals, small and medium businesses and enterprises to cre...
In addition to 22 Keynotes and General Sessions, pick from 40 technical sessions in two tracks: (1) DevOpsSUMMIT (2) Cloud-Native & Serverless. EXPIRES AUGUST 31, 2018. Ticket prices: ($295-Aug 31) ($395-Oct 31) ($495-Nov 12) ($595-Walk-in) Does NOT include lunch. DevOps Institue Certification DevOps Institute Two-Day DevOps Certification Program EXPIRES AUGUST 31, 2018. Ticket prices: ($995-Aug 31) ($1,095-Oct 31) ($1,195-Nov 12) ($1,395-Walk-in)
Despite being the market leader, we recognized the need to transform and reinvent our business at Dynatrace, before someone else disrupted the market. Over the course of three years, we changed everything - our technology, our culture and our brand image. In this session we'll discuss how we navigated through our own innovator's dilemma, and share takeaways from our experience that you can apply to your own organization.
Today we introduced our New York & Silicon Valley combo sponsorship and exhibit opportunities with unmatched pre and post-show promotion. At CloudEXPO | DevOpsSUMMIT | DXWorldEXPO NY & CA, Digital Transformation (DX) is a major focus with the introduction of DXWorldEXPO within the program. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune...