Simplified Angular Change Detection

Simplified Angular Change Detection

A detailed guide to the core part of Angular Framework

Angular is one of the most extraordinary javascript frameworks available, because it all the features required for the web development out of the box. The change detection system is at heart of framework, it essentially helps to update bindings on a page. In this article, we will learn about change detection in detail, in a simple and easy to understand way.

Before we begin, let me highlight what principles frameworks or libraries to build applications these days.

General Application Architecture

Basically what we have is the state of our application, that we are trying to replicate on the UI, this is why we need data bindings on the template. Afterward we wire up “state + template” and replicate data on view. Also in the future, any changes that happen in the state get reflected on view.

This process of syncing HTML with the state can be termed as “Change detection”, each framework has its own way of doing it. React uses virtual DOM with reconciliation algorithm, Angular uses change detection, etc. This article will cover how change detection works in Angular.

What is Change Detection?

In simple word:- a process to synchronize the model/state changes to the view.

Let’s take a simple example. Suppose we have a simple component with its own HTML like shown below.

<button (click)="title='Changed'">
Change Title

<button (click)="title='Changed'">
Change Title

app.component.html has a simple app.component.html which is displaying the app.component.html property of a component and there is a app.component.html on click which will modify the app.component.html property value of app.component.html.


When the page gets happens, it shows "app.component.html“ on the page. Later when we click on Change Titleapp.component.html it changes the binding value to ”app.component.html“. This is super awesome, the Angular framework manages this functionality for us. What it essentially does is, track the value changes which are automagically reflected on the UI. Awesome! Check this stackblitz of the above example.

Wondering 🤔 How angular updates the binding? That’s okay! Nothing really happens with magic, a framework must be running some code behind the scenes to do it. This process of updating binding is called as change detection. But the question is when angular running a change detection and how? To find out answers to these question, let’s dig a bit further.

When should change detection happen?

The simple answer to this question would be “as soon as an application’s state changes”. But when does the application’s state can change? 🤔

  1. Event Callback
  2. Network Call (XHR)
  3. Timers (setTimeout, setInterval)

Do you see any similarity in the examples above? Yes! they are all asynchronous. That means we can simply say any asynchronous call can cause a change in application state and that’s the instance where we should update our state on UI. So far so good!

Suppose we’re building our own change detection system, we would be firing change detection after the above 3 situations.

Let’s try to implement our own change detection system.

We’re just making sure to call app.component.html method from each method which includes XHR call, Timer, and Event. Just assume that the app.component.html method is responsible for the actual change detection. The vague implementation of change detection would look like the example below

<button (click)="title='Changed'">
Change Title

And the implementation of app.component.html method would look like the example below

<button (click)="title='Changed'">
Change Title

Ah! but doing this thing inside our real-world application will mess up everything. Generally in a real-world application you could have this in hundreds of thousands of places. So what can be a better implementation of this? Basically, we can also say that we are supposed to fire change detection when VM turn is over.

ZoneJS to the rescue

ZoneJS is an API which has been mostly ported out from the dart language. Behind the scenes, ZoneJS monkey patches the function. Basically, it helps to keep an eye on all async tasks, and provide an ability to call code before or after a task has been completed. The Zone API has different hooks to place your code app.component.html, app.component.html, app.component.html, app.component.html.

<button (click)="title='Changed'">
Change Title

So Angular uses the power of Zones to fire change detection. What happens is, if any asynchronous call happens, ZoneJS API emits data to the app.component.html observable, and angular calls the app.component.html method based on the same.

What happens when Application Bootstrap?

When an angular application bootstraps, it creates a platform for a bootstrapped angular module. It creates an ApplicationRef for a whole module. Basically, app.component.html has a reference to allapp.component.html, app.component.html and app.component.html (zone flag), also it has methods like app.component.html , app.component.html , app.component.html etc. You can look at this line from the source code.

Let’s have a quick look at application_ref.ts from the angular source code. You will see that after creating an app.component.html , it places a subscription on the app.component.html observable, so as soon as the VM tick over it emits a value into app.component.html observable, that will be listened to by a subscription, eventually that will call the app.component.html method inside the current Zone of application.

<button (click)="title='Changed'">
Change Title

Let’s look into how the app.component.html method works.

<button (click)="title='Changed'">
Change Title

Implementation of the tick method looks pretty simple. It basically loops over each view (components are internally referred to as view) and calls their app.component.html method which is responsible for updating UI bindings. The interesting thing is on the 4th line, it runs only in dev mode because in the application_ref.ts constructor it sets toapp.component.html property.


Above is a gist about how change detection works, let's dig deeper how we can use this while crafting Angular application.

Change Detection Strategy

In total angular has two flavours of change detection.

  1. Default
  2. OnPush

Let's look at each change detection strategy.


When you don’t specify the change detection strategy on the Component decorator, by default angular applies the app.component.html strategy.

Any Angular application consists of components, the component where we bootstraped a root component. and we can draw out a diagram of an application with regards to a component. So if change detection fires in any component it will cause the app.component.html method to be fired in the app.component.html . Ultimately firing app.component.html method from root component to its descendants as shown in the diagram below.

The problem with the default strategy is, changes that have been detected in on any component lead to firing change detection on all the components (if all component are set to app.component.html strategy. But most of the time we don’t need such behavior, it would eventually affect the performance of an application by running multiple unnecessary change detection cycle.

Can we solve this problem efficiently? Luckily there is a solution, you can easily switch the change detection strategy to app.component.html .


app.component.html strategy makes component change detection bit smarter. It runs a change detection for a component only when app.component.html bindings value of a component is changed. Actually, it compares the reference between oldValue and newValue of a app.component.html binding. That means if a parent components properties of an object do not change it would not trigger change detection for that component. It’s recommended to use app.component.html strategy for all components, resulting in a significant amount of performance gain.

As you can see in the above diagram, we’ve set a level 1 component to app.component.html strategy. Both the component have the input of app.component.html . The root component is passing the input app.component.html to both the component with app.component.html and app.component.html respectively. On initial page load change detection fires on all the components. And later the right-hand side component emits an event, which tends to change the root component state.

So, again change detection started firing from the root component. Then change detection runs for the 1st level (app.component.html) component. Before firing change detection on those components, it checks for the input binding app.component.html newValue and oldValue and if there are changes then only fire change detection for that component and it’s descendants. Changes have been detected for the right-hand side component. So change detection gets triggered only for right-hand side branch of components. By setting app.component.html change detection strategy we can significantly improve application performance.

Make sure you enforce immutability on the app.component.html binding value while using component app.component.html strategy.
Before proceeding we can have a look at a real application that is built using both change detection strategies. This application is deployed on .

This is a pretty simple application, it has posts displayed on the page and each post can have comments. The way that the application architected, it’s component hierarchy shown below.

root => post-list (all posts)=> post (single post)=> comment-list

The black border around the component indicates the boundary of that particular component.

Since we want to keep eye on when change detection fires, we highlight the components to yellow as soon change detection fires. We have used app.component.html which tells us that the change detector has visited the current component.

<button (click)="title='Changed'">
Change Title

So you can see in the above diagram, on initial page load gets highlighted and they applied with app.component.html class. Thereafter while adding a comment in a comment field, you can see that on each app.component.html event it fires change detection and all component get highlighted.

Now, look at the uses of the app.component.html change detection strategy, how it makes a difference in the change detection run cycle.

Right now all the components are set to app.component.html strategy. So on page load change detection runs for all components, that’s perfectly fine. Thereafter when I tried to add a text in the comment section, it fires change detection for the comments section of the current component, not others. That’s great! But you can see that the other components post components are getting highlighted.

Ah! What is going on? PostComponent is already set to app.component.html , there is no app.component.html binding on that component but it seems to be changed. Is this a bug, is the ngAfterViewChecked lifecycle hook being called without reason? Perhaps.

Let’s not get confused. We can look into further.

Refer to the above diagram — When change detection is running for a parent component, it follows a certain process. Initially, it updates the binding of child components, then calls the app.component.html , app.component.html , app.component.html lifecycle hook of the child component. We can also state that this process happens before rendering the parent component. Then it updates the DOM on the current component. Later it runs change detection for a child component(depending on the strategy), followed by calling hooks app.component.html , app.component.html .

That means while running change detection of a parent component, it runs app.component.html, app.component.html, app.component.html lifecycle hooks of child component irrespective of a component change detection strategy.

Check this Github issue link logged by Pascal Precht ʕ•̫͡•ʔ

You might have missed, **There is a catch. **If you look at the change detection strategy of app.component.html, it has been set to app.component.html strategy, but there is no app.component.html binding passed to it. So when app.component.html component retrieves a data form the app.component.html hook, it doesn’t run change detection from the root component (app.component.html). But it prevents to running change detection on app.component.html since no app.component.html have been changed. So we had to call either the app.component.html method or the app.component.html method of app.component.html dependency. This will force change detection to run throughout. Such cases can easily happen in a real-world application. You can tackle such situations by calling app.component.html or app.component.html .


The difference between calling app.component.html , app.component.html and app.component.html would be

app.component.html — Once you call app.component.html method on component change detector, it will traverse a component tree till root, and mark those components to run change detection only for the next iteration. It will run change detection on marked component even though they are using theapp.component.html strategy.

app.component.html — When you call this method on app.component.html provider, it will run change detection from the current component and all it’s descendants. While running change detection it keeps the change detection strategy in mind.

app.component.html** — **The app.component.html method applicable on app.component.html API. It will run change detection from the root component to all its descendants. It respects the change detection strategy of a component.

There are two more methods that exist in the app.component.html provider

app.component.html** — **By calling this method you can pluck out a component from the tree of the current component to its descendants. Whenever there is a need for running change detection on the component you could call app.component.html or app.component.html method depends on your need.

app.component.html** — **A Plucked a component from the tree can be easily brought back to its original place by calling app.component.html method. This can be used for fine tune application performance.

I hope this article has helped you to understand the mystery of what’s under the hood of change detection. Eventually, this will also make you comfortable with predicting when change detection runs in your application. By applying various flavors you can easily gain performance benefits in your application.

Node vs Angular : Comparing Two Strong JavaScript Technologies

Just from being a simple client-side scripting language, JavaScript has evolved over the years to turn out to be a powerful programming language. Here Node.js is a cross-platform runtime environment while AngularJS is one of the top JavaScript framework. Angular helps the developers to build web applications which are dynamic in nature using HTML template language and following the MVC design pattern.

An Angular Roadmap — The Past, Present, and Future of Angular

An Angular Roadmap — The Past, Present, and Future of Angular

✅Interested in being an Angular developer in 2019? ... blog post it's most likely that you've written some code in javaScript in the past.

Paleolithic JavaScript — SproutCore

In the beginning, there was SproutCore. It was the first comprehensive JavaScript framework aimed at making it easy to build desktop-quality single-page web apps. It’s not that this wasn’t possible before. When Google released Gmail, it showed the world that web apps really could replace complex desktop applications. Google even open-sourced the Closure toolkit — a set of libraries and an optimizing compiler that it used to build Gmail.

The problem was that Google’s Closure tools weren’t very developer-friendly. They relied heavily on Java, which alienated web developers who were used to working with JavaScript, PHP, Ruby, and Python. Gmail was a great demonstration of what was possible, but developing similar applications still felt out of reach for many.

Some courageous developers managed to string together amazing single page apps using a combination of jQuery, duct tape, and hope. While these apps looked amazing to end-users, for the developers working on them, the apps quickly turned into hulking piles of technical debt that made the dev team dread heading to work in the morning.

As a result, a few enterprising developers began to work on frameworks that would bring Gmail-like apps within easy reach of web developers everywhere. SproutCore was the first of these frameworks to take off. It came with a complete set of widgets that made it possible to build complex web applications without even touching HTML or CSS.

This ended up being great for former desktop developers who had been dragged kicking and screaming onto the web. Several more frameworks popped up with similar goals; GWT and Cappuccino were the most prominent. These frameworks even avoided JavaScript by transpiling other languages into JS. Again, this was great for desktop developers. But it left passionate web developers out in the cold and made them feel as though their hard-won HTML, CSS, and JavaScript skills weren’t valuable.

This left an opening for a framework that truly embraced the web, instead of trying to plaster over it and pretend it was something else. A couple of early frameworks (Backbone and Knockout) appeared, and achieved a moderate amount of success. Ember also showed up around this time. It took SproutCore, stripped it down to its bones, and tried to rebuild it into something truly web-friendly. Ember wanted to be the Six Million Dollar Man of the JavaScript world: rebuilt better, stronger, and faster.

None of these frameworks rocketed to popularity. The world was waiting for something better. In 2010, that something better appeared — it was named Angular.

The Golden Age of Angular

Even before Angular version 1.0 had been released, Angular took the front-end development world by storm. Finally, we had an easy-to-use JavaScript framework that treated HTML as a first-class citizen. Developers and designers could now work together to build amazing single-page applications. This came as a relief to designers, who had been annoyed and offended because older frameworks had treated HTML and CSS as tools for barbarians, tools that no civilized developer should have to touch.

The first thing that seemed magical to developers trying Angular for the first time was two-way data binding. Prior to this, most developers had only seen this kind of data binding in desktop frameworks like WPF and Windows Forms. It was great to be able to bind forms and other UI elements to JavaScript model objects. While two-way data binding could cause performance problems when overused, teams that used it judiciously found that Angular enabled them to create complex front-end applications much more quickly than ever before.

Angular proved to be popular for more than just easy binding of data to HTML elements. Angular directives provided an easy way to create reusable HTML + CSS components. Although other JavaScript frameworks provided this before Angular, Angular was the first one that became extremely popular. Reusable components had long been in-use in server-side frameworks. ASP.NET user controls and partial templates in Rails and Django are but a few examples.

Finally, Angular made dependency injection popular in the front-end development world. Dependency injection had long been popular in enterprise applications, which is perhaps why it hadn’t caught on in the JavaScript world. Front-end developers have long been averse to what they see as needlessly complex enterprise software design patterns. This concern isn’t without merit. Have you ever, in the course of writing an application, said to yourself “What I really need here is a “SimpleBeanFactoryAwareAspectInstanceFactory?”

Dependency injection, though, has proven its worth. And Angular made dependency injection easy to use for an audience that hadn’t used it much in the past. Need an HTTP client? Or perhaps you’d like to do some animation? No problem. Angular had built-in services for those. Just ask for them, and Angular would inject them into your components. No need to instantiate anything yourself.

Or perhaps you wanted to use the browser’s window or location objects without making it impossible to unit test your components outside of a browser? Angular had you covered there too, with its built-in $window and $location services. At runtime, you’d get the browser objects you were expecting. And when running unit tests server-side in Node.js, you could pass mock services into your components to ensure they behaved as expected in various scenarios.

If all of this wasn’t enough, Angular also made it simple to register and inject your own services. For developers who were used to binding all their data to the DOM and hoping for the best, this was awesome. If you were writing a new front-end app that called for APIs that would cost your company a lot of money if overused, you’d probably prefer to be able to write tests ahead of time to ensure that your application doesn’t try to do something like calling the Twilio API 800 million times.

So you’d create a Twilio service that gets injected at runtime. At testing time, you’d create a mock service that records the cost of every API call your application is trying to make. You’d write tests that cover common usage scenarios and ensure that these scenarios don’t result in your company receiving a 7-figure bill. Overall, most developers found that Angular directives combined with dependency injection made it possible to write modular, testable front-end applications using tried-and-true software engineering techniques. Many development teams decided that this resulted in a happy state of affairs, and decided to go all-in on Angular.

The Angular Decline? The Rise of React

While things were mostly great in the world of Angular, it wasn’t all sunshine and lollipops. Developers were starting to run into severe performance problems when they tried to bind too many model objects to too many DOM elements. Some applications slowed to a crawl. Direct calls to $digest and other black-magic sorcery started becoming necessary to achieve acceptable performance. Around the same time, a new challenger appeared: React. At first, React didn’t seem to pose too large a danger to Angular. After all, these React weirdos had gone to the trouble of inventing JSX, which looked a lot like a way to mix markup into your code. Hadn’t we gone to a lot of trouble to invent templating languages for the explicit reason of avoiding mixing markup and code?

As it turned out, React’s approach was pretty popular in the front-end development community. It didn’t rocket to popularity, however. Angular was still dominant, and it looked like it would remain that way. Until that is, Angular’s popularity was given a good kick in the teeth from an unexpected source: the Angular team itself.

The Introduction of Angular 2

Angular 2 was first announced at the ng-europe conference in 2014. The Angular team’s plans came as a shock to the Angular community, to say the least. Reaction from Angular developers was swift and negative… and not without reason. Angular 2 would be getting rid of many familiar concepts from Angular 1, introducing a new, incompatible templating language (and oh, by the way) would also be programmed using an entirely new language.


Although both Angular 1 and Angular 2 were called ‘Angular,’ in reality, they were very different frameworks with a few things in common. To help prevent confusion, the Angular team started referring to the old version of Angular as ‘AngularJS’, and the new version as simply ‘Angular.’ This makes intuitive sense since AngularJS was written in JavaScript, and Angular was not. To keep the distinction between the frameworks clear, I’ll be referring to Angular 1 as AngularJS from this point forward.

As a result of all of this, AngularJS developers lost faith in the framework’s future. They threatened to move to a new framework on future projects, and that is precisely what many of them did. React was the biggest beneficiary of the mass exodus from AngularJS.

Although React didn’t do as much as AngularJS, in a way that was positive. If you’re using a view library that doesn’t try to include everything plus the kitchen sink, it’s a lot more difficult for the developers of that library to pull the rug out from under you in the future. In the beginning, using React was a bit of a pain compared to AngularJS. You had to cobble together a patchwork of libraries just to cover the functionality the AngularJS provided out of the box.

Many teams saw this as a good way to reduce risk, because it was unlikely that the developers of all of those libraries would decide to make backward incompatible breaking changes at the same time, which is essentially what Angular had done.

The Emergence of Vue

To compound AngularJS’ woes, another framework named Vue showed up at about the same time the drama over Angular 2 was occurring. Vue was inspired by AngularJS but aimed to simplify it and get rid of what Vue’s creator saw as unnecessary complexity (so Vue felt very familiar to existing AngularJS developers). Vue provided a safe haven for many AngularJS developers who didn’t want to move over to React.

This doesn’t mean that AngularJS developers were not waiting patiently for Angular 2 to appear. But it’s clear that there was a mass exodus from AngularJS to React and Vue due to the uncertainty caused by the plans for Angular 2.

Rising From the Ashes with Angular 2

Eventually, Angular 2 was released. As expected, it did away with many familiar concepts from AngularJS but kept a few of the best pieces like services and dependency injection. Fortunately for the sanity of developers, Angular uses plain TypeScript and not a fork as originally planned.

To make things more confusing, the Angular team maintained a fork of the new framework that used the Dart programming language instead of TypeScript. Initially, the TypeScript and Dart versions were developed in sync, generated from a single code base. Eventually, the TS and Dart versions of Angular decided to go their separate ways, and Angular Dart now exists on its own.

Even with this confusion, Angular’s popularity began to increase again after the Angular 2 release. It happened slowly. As often occurs in software development, trends shifted. Developers realized that a big, batteries-included framework might actually be useful. After all, when your application grows large enough, you end up actually needing all of those batteries.

Enterprise developers, in particular, began moving back to Angular. This makes sense. Usually, when you start an enterprise web app, you know it is going to be complex. There’s no point in starting with a tiny MVP when you know from the beginning all 87 things your application is going to be expected to do.

Where’s Angular 3?

Although Angular 2 wasn’t perfect, many developers of complex web applications began to realize that the new-and-improved Angular was a good fit for their needs. Fortunately for them, Angular had some exciting improvements in store. More importantly, the Angular team demonstrated that it could consistently publish new versions of the framework with few breaking changes between versions

In a move that seemed odd at the time, the Angular team decided to skip version 3 entirely and move to version 4. This was done for good reason: the team working on Angular’s router package had already pushed ahead and released version 3, while the remainder of Angular was still at version 2.3. They decided to keep all Angular package versions in sync moving forward, and bumping everything up to version 4 for the next release was the easiest way to achieve this.

Angular 4

Angular 4 had some significant changes, including added ahead of time compilation, which resulted in small production JavaScript bundles and shorter application load time. Support for server-side rendering was added, which was a boost for teams that wanted to render their app ahead of time to improve initial load performance. Many other improvements were added throughout the framework, but upgrading apps from Angular 2 to 4 was quick and painless in most cases.

Angular 4.3 and Angular 5

Angular 4.3 added a new HTTP client that was easier to use than the old HTTP service. In Angular 5, the old HTTP service was deprecated and would be dropped in the next release. In spite of this inconvenience, there was relatively little grumbling because the upgrade in most cases was straightforward. Angular 5 also added better internationalization support and further build optimizations.

Angular 6 and 7

Angular 6 and 7 were disappointing to some developers. There were no large changes, but there were many small quality of life improvements, especially to the Angular CLI tooling. The decreasing number of visible changes isn’t an indication that the Angular team has stopped innovating. Instead, it shows that the framework is mature, so the development team is now free to do more work behind the scenes, fixing bugs and improving performance.

The stability of the framework since the release of Angular 2 has drawn some old-school AngularJS developers back into the Angular world. It has also attracted the attention of enterprise development teams. When you’re building enterprise apps that may live for decades it’s ideal to use a framework that gets new releases on a predictable schedule but doesn’t change too rapidly. A developer who had only used Angular 2 could be up and running and contributing to an Angular 7 app within minutes.

The Future of Angular

Angular 8 and Angular Ivy

And that brings us to today. As we’ve seen, Angular has come a long way. It has gone from loved by web developers to being reviled to being admired, although it isn’t yet loved again like it was in its early days.

On the horizon, we have Angular 8. A ton of work has been done in Angular 8 to make it easy to use with the Bazel build system, which is absolutely amazing news for all 3 developers who are using it outside of Google. More excitingly, though, the Angular team is hard at work on a new rendered called Angular Ivy. It’s intended to be a drop-in replacement for the current rendered. For the most part, current apps won’t need to make any changes to use Angular Ivy.

If Angular Ivy is a drop-in replacement, why should developers care? Two important reasons: speed, and bundle size — two very important concerns. For a few years, it seemed like web developers had gone a bit crazy. Teams were shipping JavaScript bundles that were 5MB, 10MB, or even larger, and thinking that there was no problem with this. After all, the applications worked perfectly on the developers’ i7-powered MacBook Pros so they should work fine for everyone, right?

Unfortunately, in the real world, not everyone is running the latest and greatest hardware. Hundreds of millions of people access the internet solely on older Android phones with slightly more processing power than a potato, through internet connections only a little faster than dial-up. For these users, a huge JavaScript bundle takes forever to load, and even longer for their device to parse and run. Even in less extreme cases, there are countless users around the world who aren’t using the latest and greatest hardware. For them, massive JavaScript apps are usable (but painful).

Angular Ivy Expectations

The Angular Ivy renderer will help in several ways:

  1. It is being written with an eye on efficiency, so it will accomplish the same tasks while executing far fewer CPU instructions. This will improve both the battery life and the sanity of users with less-than-powerful devices.
  2. The renderer will be written in a much more modular fashion that the current renderer. This will make it much more amenable to tree-shaking and dead code elimination. As a result, your production JavaScript bundle will include only the code that is needed to run your application, instead of bundling together everything plus the kitchen sink as often happens with the current rendered.
  3. In addition to the bundle-size reduction and improved rendering speed, Angular Ivy has another few quality-of-life enhancements for Angular developers. Rebuild times are significantly faster. So if you’re running your app in development mode and waiting for your changes to appear, you’re now going to be spending a lot less time waiting.
  4. Template-type checking is improved, which means you’ll catch more errors at compile time instead of at runtime. Runtime template bugs are annoying, because they either bite you during testing, or they bite your users when they’re trying to use your app.
  5. The Angular Ivy template compiler will generate code that is human readable, which the current View Engine compiler doesn’t do. This will come in handy when trying to track down tough template bugs.

The net result: smaller apps, faster apps, happier developers, and happier users.

Angular’s Past, Present, and Future

If you’ve been using Angular from its early days all the way until now, then congratulations! While there have been plenty of rough patches, we’ve ended up with a fast, modern framework that is fun to use.

If you were an AngularJS developer but moved on to React, Vue, or something else, I encourage you to give Angular another look. It’s worth your time, even if you decide to stick with what you’re using now.

And if you’ve never used Angular at all, why not give it a shot?

We’ve just been on a whirlwind tour through Angular’s past, present, and future. Without a doubt, it has been quite a ride. Regardless of your Angular background, I hope you’ve enjoyed the tour!

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