Learn how to Optimize Template Expressions in Angular

Learn how to Optimize Template Expressions in Angular

Learn how to Optimize Template Expressions in Angular . In this post, we will look at template expressions in Angular and learn how to optimize them to speed-up performance Template expressions were

Template expressions were, for me, one of the most useful features when I started using Angular in 2017. Define your data /property in your component and render it on the component using your double curly braces — that was awesome. Though if found it very useful, I still needed to know the best practices around using template expressions in Angular.

In this post, we will look at template expressions in Angular, and how to optimize it to top up our speed.

Template Expression

Let’s start with an example:

@Component({
  selector: 'app-chat',
  template: `
    <div>
        {{viewRun}}
        <h1> Count Component {{count}} </h1>
        <button (click)="onClick($event)">Click Me</button>
    </div>
  `,
  style: []
})
export class ChatComponent implements OnInit/*, DoCheck, OnChanges*/ {
    count = 0
    onClick(evt) {
        this.count++
    }
  get viewRun() {
    console.log('view updated:', this.count)
    return true
  }
}

We have two bindings in our template: viewRun and count. This tells Angular that the field in the template is dynamic, that is, it will be dynamically updated during CD run by Angular.

count is a property of the ChatComponent and viewRun is a method of the ChatComponent. When a Cd is run on the ChatComponent, Angular gets the current value of the count property and replaces it with the {{count}} in the template. viewRun, being a function, is run by Angular and the returning value is replaced with the {{viewRun}} expression in the template.

You see why they are called bindings. They glue the properties/methods of the class component to the view. Each time the Click Me button is clicked, the count property is incremented by 1. The increment is now updated in the view {{count}} on every CD run.

CD run in Angular is triggered by:

  • async events:
  • DOM events e.g. click, changes, mouseover, etc.
  • setTimeout, setInterval, etc.
  • Implicit CD call by the component.
Performance Bottleneck on Template Expressions

Whenever a Cd is run on the ChatComponent, the bindings are run and updated. The bindings in the component, especially the function viewRun, are called and the result is rendered on the DOM during DOM interpolations.

We are actually calling a function on the template of the ChatComponent component. What will happen if the function ‘viewRun’ doesn’t finish quickly? The users will experience drag or slowdowns.

For demo purposes, let’s make the ‘viewRun’ function execute very slowly, like to take several seconds before it finishes.

function viewRun() {
    function wait(ms) {
        let now = Date.now()
        let end = now + ms
        while (end > now) {
            now = Date.now()
        }
    }
    wait(20000)
}

See, we made the viewRun run in 20 secs!! That’s a huge performance impairment. So the users will have to wait for> 20s to see something render on the screen.

Best practices in Angular warns us not to compute heavy operations in the template, we should only execute functions that finish immediately.

Optimize Template Expressions

Let’s look at another example:

function factorial(num: number) {
    if (num == 1)
        return 1
    return num * factorial(num - 1)
}
@Component({
  selector: 'app-users',
  template: `
      <div *ngFor="let user of users">
        {{user.name}}
        {{user.id}}
        Factorial of User ID {{factorial(user.id)}}
      </div>  
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
class Users {
  @Input() users: Array<User> = []
}
@Component({
  selector: 'app',
  template: `
    <app-user [users]="users"></app-user>
    <button (click)="addToUsers($event)">Add A User</button>
  `
})
class App {
  users: Array<User> = [
    {
      id: 99,
      name: "Nnamdi"
    },
    {
      id: 88,
      name: "David"
    },
    {
      id: 77,
      name: "Philip"
    },
    {
      id: 66,
      name: "Chidume"
    }
  ]
  lastId = 66
  addToUsers() {
    this.users = this.users.concat([
      {
        id: this.lastId++,
        name: "User #ID: " + this.lastId 
      }
    ])
  }
}
interface User {
  id: number;
  name: string;
}

We have three components, Users and App components. In the Users component, it lists a users data stored in an array using the *ngFor directive. It displays the name, id and the factorial number of the user's id. The Users component is an OnPush component, it is a "pure" component in the sense that Angular updates the component when the input bindings in the component change. When we click the Add A User button a new user is added to the users array, this causes the Users component to re-render to display the new user.

Now, for every user added to the users array, the factorial template expression for all the users. Now, imagine we have about 1000 users in our array and displayed on the screen and we clicked to add another user to the array. The factorial template expression will be run for the 1001 users !!! We all know that calculating the factorial of numbers can become CPU intensive.

Although we have optimized our component not to run on unnecessary CD triggers yet we have issues in the template expressions. Yes, we can’t do away with the factorial template expressions but we will find a way of making execute less when a new user is added to the array.

We will use Caching/Memoization to make our factorial function execute less frequently. What does it mean?

Our factorial function is straight-forward and doesn’t have side-effects, what you give it, it produces an output based on the input. We can memoize it, by making it store the results of each input and return on subsequent calls when the factorial function is called with the same inputs.

Example:

In the factorial, to find a factorial of a number, it’s multiplied subsequently by its difference.

2! = 2 * 1
4! = 4 * 3 * 2 * 1
6! = 6 * 5 * 4 * 3 * 2 * 1
10! = 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2 * 1

Look at 4! = 4 * 3 * 2 * 1, do you know we can only calculate for 4 * 3 and return 2! its prev calc. Instead of calculating it all over again whereas we have already done that earlier.

4! = 4 * 3 * 2!

That’s memoization, instead of calc a new value afresh we simply look it up to know whether it has already been calculated previously and get the result from there.

Lets memoize the factorial function:

function factorial(num) {
    if (!factorial.cache) {
        factorial.cache = {}
    }
    if (!factorial.cache[num]) {
        if (num == 1 || num == 0)
            return 1
        factorial.cache[num] = num * factorial(num - 1)
        return factorial.cache
    }
    return factorial.cache[num]
}

Wow!! lots of code. All we did here is just look a num in the cache factorial.cache object, if it's present the result is returned if not the factorial is performed and the result in the cache and returned.

Let’s test it. We will call 2! first followed by 4!. In 2! call, We would expect to have cache misses for 2 and 1 because it is the first time of calling with the inputs. Then, in 4! we would expect cache misses for 4, 3 but a cache hit for 2.

// ...
function factorial(num) {
    if (!factorial.cache) {
        factorial.cache = {}
    }
    if (!factorial.cache[num]) {
        c.log("cache miss: ", num)
        if (num == 1 || num == 0)
            return 1
        factorial.cache[num] = num * factorial(num - 1)
        return factorial.cache
    }
    c.log("cache hit: ", num)
    return factorial.cache[num]
}
c.log("2! call")
factorial(2)
c.log("\n")
c.log("4! call")
factorial(4)
[email protected] MINGW64 /c/wamp/www/developerse/projects/trash
$ node csl
2! call
cache miss:  2
cache miss:  1
4! call
cache miss:  4
cache miss:  3
cache hit:  2

Perfect!! like we expected. If we apply this to our Users component we will greatly minimize the rate of factorial deep calculations, don’t get it twisted, the factorial function will always be called when the input bindings changed and CD is triggered, we just optimized it not to do unnecessary calculations that it had previously done, only for the new user added.

function factorial(num: number) {
    if (!factorial.cache) {
        factorial.cache = {}
    }
    if (!factorial.cache[num]) {
        c.log("cache miss: ", num)
        if (num == 1 || num == 0)
            return 1
        factorial.cache[num] = num * factorial(num - 1)
        return factorial.cache
    }
    c.log("cache hit: ", num)
    return factorial.cache[num]
}
@Component({
  selector: 'app-users',
  template: `
      <div *ngFor="let user of users">
        {{user.name}}
        {{user.id}}
        Factorial of User ID {{factorial(user.id)}}
      </div>  
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
class Users {
  @Input() users: Array<User> = []
}
// ...

Using Pure Pipes

We can do the same using pure pipes. What are pure pipes? Pure pipes return the same output for particular inputs, they do not have side-effects on the program. Pure pipes evaluate a given expression only if they have been giving a different set of inputs from the prev. ones.

Examples of pure pipes in Angular are Decimal pipe, Number pipe, Percent pipe, Currency pipe, and Date pipe.

These pipes are pure because:

  • They do not have side-effects
  • They return a specific output for a specific input.

Taking the Date pipe as an example

{{ 10 June 1985 | date: 'shortDate'}}
6/10/85

Now, the first time we passed in 10 June 1985, Angular will evaluate the expression but on subsequent calls with the same input, the expression will not be evaluated but the prev. value will be returned because the arguments haven't changed.

Let’s make our factorial function into a pure pipe:

function factorial(num: number) {
    if (num == 1)
        return 1
    return num * factorial(num - 1)
}
@Pipe({
  name: 'factorial',
  pure: true
})
class Factorial {
  transform(val: number) {
    return factorial(val)
  }
}

See we moved the uncached version of our factorial function inside a pure pipe, we will no longer need our cached version Angular will provide it for us for free!! :)

We can now remove the {{ factorial(user.id) }} template expression and replace it with {{ user.id | factorial }}.

Using Decorators

Decorator is a new feature in TypeScript introduced to add additional info to a class, method, property during runtime. It is denoted using the @ symbol.

Angular made heavy use of Decorators:

  • @Component
  • @Directive
  • @Output
  • @Input
  • @Optional
  • @Self
  • @Injectable
  • @NgModule
  • @Pipe
  • … etc

We can use the decorator to add caching mechanisms in our app.

function memoize(fn: any) {
    return function () {
        var args =
Array.prototype.slice.call(arguments)
        fn.cache = fn.cache || {};
        return fn.cache[args] ? fn.cache[args] : (fn.cache[args] = fn.apply(this,args))
    }
}
function purify () {
  return function(target: any, key: any, descriptor: any) {
    const oldFunc = descriptor.value
    const newFunc = memoize(oldFunc)
    descriptor.value=function() {
        return newFunc.apply(this,arguments)
    }
  }
}

We have our method decorator purify, it returns a function that takes three params: target, the class of the method in which the class will be decorated with; key, the name of the method which is decorated; descriptor, an object containing the function/method in its value property which is decorated.

In the implementation we save the original function in the oldFunc variable, then we memoize it by passing the oldFunc to the memoize function. The memoize function is a general function used to memoize functions. The returned memoized function is saved in the newFunc variable. Next, we assign a new function to the descriptor.value property, the function implementation calls the newFunc (the memoized version) and returns the value.

To test it out, we would create a mock class with a factorial method and decorate it with the purity decorator.

class PureClass {
    @purify()
    factorial(num: number): number {
        console.log("cache miss: ", num)
        if (num == 1)
            return 1
        return num * this.factorial(num - 1)
    }
}

See we decorated the factorial method with @purify(). Let's run it:

const p = new PureClass()
console.log(p.factorial(2))
console.log(p.factorial(4))
[email protected] MINGW64 /c/wamp/www/developerse/projects/trash/di-ts
$ ts-node mem
cache miss:  2
cache miss:  1
2
cache miss:  4
cache miss:  3
24

See we have a cache miss for 2 because it is the first time it is called on subsequent times it will be returned from cache. For 4, we have a cache miss for 4 and 3 and a hit for 2 because 2 have been cached previously. If we call for 6 we will have a cache hit for 4,3,2 but not for 6 and 5 because it is the first time the function is seeing it.

Now, let’s apply it to our Angular class component:

@Component({
  selector: 'app-users',
  template: `
      <div *ngFor="let user of users">
        {{user.name}}
        {{user.id}}
        Factorial of User ID {{factorial(user.id)}}
      </div>  
  `,
  changeDetection: ChangeDetectionStrategy.OnPush
})
class Users {
  @Input() users: Array<User> = []
  @purify()
  factorial(num: number) {
      if (num == 1)
          return 1
      return num * factorial(num - 1)
  }
}
Conclusion

We learned how to optimize template expressions in this post. We saw the slowdowns that may occur if we run a high performant function in our templates. Also, we looked at different ways we can highly optimize the template expressions by caching the functions:

  • memoization
  • pure pipes
  • decorators

In the next of our Angular runtime performance checklist series, we will be looking at how to optimize *ngFor directive in our template to avoid unnecessary DOM manipulations. Till then.

If you have any questions regarding this or anything I should add, correct or remove, feel free to comment, email or DM me.

Thanks !!!

Migrating from AngularJS to Angular

Migrating from AngularJS to Angular

Migrating from AngularJS to Angular a hybrid system architecture running both AngularJS and Angular

Intro

Dealing with legacy code/technologies is never fun and the path to migration isn’t always as straight forward as you want. If you are a small startup, trying to balance business requirements, scarce resources and aggressive deadlines, it becomes even more complicated.

This is the situation one of the startups I was advising was facing.

A bit of background

The startup was developing a SaaS for the last 2 years and (at the time) had around 15 clients worldwide. In these 2 years their code base grew pretty fast and lead to quite a lot of fast/reckless written code. There was nobody to be blame, this is pretty common in the startup world when business needs move way faster than you expect and you start sacrificing code qualify for quantity.

The system architecture was pretty simple. 
• a frontend application written in AngularJS (split into multiple modules that were selected at build time depending on the clients’ configuration)
• a backend application written in Python 2.7 and Django 1.9 using a Mysql database
• Celery for running async tasks

Each client would get their own isolated environment deployed on AWS:
• Apache in front of the Django application (deployed on multiple EC2 instances behind an ELB)
• AngularJS build deployed on individual S3 buckets with CloudFront in front of them

Path to migration

A few months before starting the migration, development was getting very slow, features were not coming out as fast, deadlines were missed and clients were reporting more issues with every update that we were rolling out. It was at this time that we started thinking more seriously about some kind of refactoring or major improvement.

We didn’t know exactly what we were going to “refactor/improve” so we started off by answering three questions (I recommend that anyone who is thinking about a migration/refactoring think really hard about the how to answer them):

1st question: Why is refactoring necessary now ?

This is a very important questions to answer because it helps you understand the value of the migration and also it helps to keep the team focused on the desired outcome. For example because i don’t like the way the code is written isn’t good enough reason. The reason has to have a clear value proposition that somehow directly or indirectly benefits the customers.

For us it was mainly three things:
 1. feature development was becoming painfully slow;
 2. code was unpredictable. we would work in one part of the application and break 3 other parts without realizing;
 3. single point of failure: only 1 engineer knew the FE code base completely and only he could develop new features on the codebase (this is out of a team of only 5 engineers)

So our goal was simple:

improve FE development velocity and remove the simple point of failure by empowering other engineers to develop FE features

2nd question: Who is going to do the migration ?

You can answer this question either now or after the 3rd question. Depending on the size of the company and on the available resources it can be one person, several people, an entire team, etc…

We were in a difficult situation. The only developer who could work on this couldn’t because he was busy building critical features for our customers. Luckily we had one senior backend engineer who wanted to get some FE exposure so he volunteered to take on the task. We also decided to time-box a proof of concept at 2 weeks. We did this because we didn’t know how long it would take to figure out a solution or whether the engineer could actually do this task since he hadn’t worked on FE before.

3rd question: What are we actually going to do ? The answer here usually involves some discovery time, a few tech proposals and a general overview of the options with the entire team while weighing the pros and cons of each.

For us one thing was clear from the start: we didn’t want to invest any resources into learning/on-boarding engineers on AngularJS. AngularJS had already entered Long Term Support and we didn’t want to have our engineers invest time in something that might not benefit them long term. This meant that refactoring the existing AngularJS code was not an option. So we started looking at Angular6 …

The migration

There a multiple approaches on how to have a hybrid app running different frameworks. After reviewing some options we decided that — for us — the best way to move forward was to simply have 2 separate FE applications deployed: the legacy AngularJS one and the new Angular one. This meant that any state on one app could not be transferred to the other application, which wasn’t such a big deal for us since only new modules were going to be developed using Angular and our modules didn’t share state with each other.

From the client’s perspective everything would look like one application, except for a page reload when they would move between the applications.

Pros to this approach

  • speed: get something up and running without untangling legacy code
  • safety: no risk of breaking the current live app since it would be a new code based deployed next to the old one (especially since a developer with no previous exposure to the project was working on it)
  • stop legacy development: we stop adding more code the an already unmanageable codebase

Cons to this approach:

  • maintaining legacy code: it didn’t address feature improvements on existing modules; old modules would still be in AngularJS for an undefined period of time
  • duplicating parts of the code: since the new app had to look and feel like the old one any themes, custom components would have to be written in both places. Also some parts of the layout would have to be duplicated in new app (like header, menu, etc.. ) and any changes to those components would have to be done in both apps

We already knew of a new module that we wanted to build so we started form scratch with a new Angular 6 project and we used this new module for our 2 weeks proof of concept.

Step 1— same domain

Have both apps running on the same domain so that they have access to the same cookies and local data. This was extremely important since only the AngularJS app would continue handing authentication & authorization.

Step 2— look and feel

Both apps The goal was to make the new app look the same as the original application. So we: 
 • copied over all the stylesheets
 • implemented the base layout of the application (header & menu drawer)

Step 3 — authentication & authorization

We had to duplicate the authorization logic in the Angular6 app and make sure the correct session tokens were available to allow access to the module

Step 4— routing between apps

Since our main navigation links would take you to either app, we decided do move all that logic to a backend service called menu-service. This would eliminate the need to write any navigation changes in both apps and also would allow for greater runtime control over what navigation buttons we show.

Example:

HEADER: Authorization: Bearer xxxxx
GET menu-service/v1/menu/?type=0|1 (0: legacy, 1: new)
[{
  "slug": "refresh",
  "name" : "Refresh",
  "icon" : "fa-refresh",
  "type" : 1  
 }, {
  "slug": "module1",
  "name" : "Module1",
  "icon" : "fa-module1",
  "type" : 1
}, {
  "slug": "module2",
  "name" : "Module2",
  "icon" : "fa-module2",
  "type" : 0
}, {
  "slug": "logout",
  "name" : "Logout",
  "icon" : "fa-logout",
  "type" : 0
}]

In the above example based on the type value we identify that the module1 and refresh are links towards the new application while module2 and logout are links in the old application.
This information allows each application to decide whether to use the internal routing mechanism or do a window.location redirect

Example of routing in the Angular app (AngularJS does something similar):

export class MenuService {
  constructor(private router: Router) {  }
  onMenuItemClicked(menuItem): void {
    if (menuItem.type === 1) {
      this.router.navigate([menuItem.slug])    
    } else {   
      const url = `${legacy_endpoint}/${menuItem.slug}`;
      window.location.href = url      
    } 
  }
}

Step 5— building/deployment on a real environment

Like i mentioned in the beginning the AngularJS application was deployed to an AWS S3 bucket and exposed through Cloudfront to take advantage of the massively scaled and globally distributed infrastructure offered by AWS.

The result we wanted was the following: anything that has the url [https://hostname/v2](https://hostname/v2)/ is routed to the Angular application and everything else is routed to the legacy AngularJS app.

We used base-href and to make sure our Angular6 application builds accordingly

ng build --base-href /v2/ --deploy-url /v2/

Unfortunately we didn’t manage to achieve the desired routing behavior with AWS Cloudfront. This was a big disappointment since we had to pivot to a less optimal solution. (if anyone has any suggestion on how to do this in Cloudfront i’d love to hear it)

We ended up with the following structure:
• each app deployed in a NGINX Docker container

# AngularJS — Dockerfile:
FROM nginx:alpine
COPY dist /usr/share/nginx/html
--------------------------------------------------------------------
# Angular6 — Dockerfile:
FROM nginx:alpine
COPY dist /usr/share/nginx/html/v2

• AWS ALB with path routing

Step 6: Local development

Local development for the AngularJS application didn’t have to change. However in order to develop on the Angular6 app you had to also run the AngularJS application to be able to authenticate and get the appropriate session tokens.

We were already using Docker for deploying our application as containers. So we added a Makefile target to run the latest from our Docker repository

# Angular6 — Makefile:
AWS_REPOSITORY = xxx.dkr.ecr.eu-central-1.amazonaws.com
JS_APP_NAME = angular-js
...
run-local: 
  docker run -p 8080:80 $(AWS_REPOSITORY)/$(JS_APP_NAME):latest

Conclusion

This might not be the cleanest or optimal solution, however it was the fastest way towards our goals. And this was the most important thing to us.

The goal of this post isn’t to teach you how to do a AngularJS to Angular6 migration but instead is to showcase our path when dealing with such a task.

Further reading:

An in-depth look at Angular’s ng template

Angular 8 Data (Event & Property) Binding Tutorial: Build your First Angular App

Angular 8 Forms Tutorial - Reactive Forms Validation Example

What is the difference between JavaScript and AngularJS?

JavaScript is a client-side programming language used for creating dynamic websites and apps to run in the client's browser whereas AngularJS is a fully featured web app framework established on JavaScript and managed by Google.

JavaScript is a client-side programming language used for creating dynamic websites and apps to run in the client's browser whereas AngularJS is a fully featured web app framework established on JavaScript and managed by Google.


What’s the difference between AngularJS and Angular?

What’s the difference between AngularJS and Angular?

Angular vs Angularjs - key differences, performance, and popularity

AngularJS was released in 2009 and quickly became popular for it's two-way data binding, MVC architecture, and code reusability.

When alternatives like React and Vue delivered the same advantages of AngularJS with better performance, the Angular team decided to completely rewrite the framework.

Each subsequent release of Angular (4,5,6,7,8) has been mostly non-breaking incremental changes. For these reasons, "Angular" now refers to Angular 2+ and "AngularJS" the original.

Key Differences

Here are the key differences between Angular 2+ and AngularJS:

TypeScript

Angular was rewritten using TypeScript. TypeScript is a superset of JavaScript. It compiles to regular vanilla JavaScript but provides syntax for type checking, annotations, and ES6 based extensions.

Since TypeScript is a superset of JavaScript, it needs to be compiled or "transpiled" into ES5 JavaScript so your code still runs in the browser. This requires the use of NodeJS and other build tools for preprocessing TypeScript files.

While using Angular 2+ without TypeScript is possible, the industry standard is to adopt TypeScript as it plays much better with the Angular ecosystem.

MVC vs Component Architecture

AngularJS adheres to the model, view, controller (MVC) software design pattern. Controllers are defined with $scope variables representing an underlying data model. This data model can be updated in both the view and the controller. The view is an HTML file which both displays and dynamically updates $scope variables.

Angular 2+ utilizes more of a component based architecture. Isolated pieces of functionality are defined in components. These components reference their own templates and stylesheets and exist in a hierarchy of other components.

Dependency Injection (DI)

Both AngularJS and Angular use dependency injection. DI allows you to share commonly used functionality across different controllers or components.

In AngularJS, dependencies are injected in controller functions, link functions, and directive definitions.

In Angular, constructor functions, providers, and declarations are used to manage these dependencies.

Angular CLI

Angular 2+ features the Angular CLI: a command line interface for quickly generating Angular components, services, directives, etc. It comes with convenient commands for building your Angular project (compiling TypeScript files and other assets into vanilla js files that run in the browser). It also makes building your project for different environments easier and allows for things like linting, type checking, etc.

AngularJS doesn't have it's own CLI.

Performance

Angular is much faster than AngularJS. In fact, it's said that Angular can be more than 5X faster based on the design of your application.

Popularity

Before the advent of React and Vue, AngularJS was very popular. It offered an elegant solution to the JavaScript SPA with two-way data binding and MVC architecture.

Being able to dynamically update a JavaScript POJO from an HTML template caused a lot of buzz. As a result, alternatives like React and Vue emerged with superior diffing algorithms that left AngularJS in the dust.

Angular fought back with the release of Angular 2 (2016). Today, Angular remains one of the most popular frameworks for UI development.

While AngularJS is still used today, it's popularity has died in favor of more current options like Angular 2+, React, and Vue.

Performance

The problem with AngularJS

Performance is one of the biggest problems with the original AngularJS. This is due to the underlying "magic" behind what originally made AngularJS so popular.

To achieve two-way data binding, AngularJS relies on a digest cycle to keep views in sync with their underlying data models. It works by augmenting all event handlers (clicks, ajax, timeouts) with a process called "dirty checking". Each scoped variable is compared to it's previous value.

If something has changed, the watchers and templates are updated with the new value and the process runs again to see if anything else has changed. In this way, the view is constantly in sync with the data model.

The problem with the AngularJS digest cycle is it's unpredictable. As applications grow, the "checking" process becomes more intensive and can run infinitely with two way data flow.

Angular 2+ to the rescue

To address these issues, the Angular team rewrote the framework with flux architecture in mind. Specifically unidirectional data flow was fundamental to reengineering change detection in Angular.

Now the Angular framework is just as fast as alternatives. When compared to AngularJS, Angular can be more than 5X faster.

The Angular CLI also makes minifying production bundle sizes a breeze, keeping Angular light weight for production.

Advantages of Angular

Angular offers many advantages over the original AngularJS:

Performance:

Angular is up to 5X faster than AngularJS. This is because of a superior diffing algorithm featuring unidirectional data flow and component based architecture.

Server side rendering

Angular offers extensions for rendering your application server side. This is huge for SEO as certain web crawlers can't always scrape async content.

Mobile development

As a framework, Angular makes it possible to develop applications that work on both browsers and native devices like iOS and Android.

Lazy loading

Lazy loading allows you to asynchronously load JavaScript components based on route. This can offer additional performance advantages as code is only imported when it's being used.

Tooling

The tooling provided by TypeScript and the NodeJS ecosystem can't be underestimated. Using the Angular CLI, you can quickly generate Angular components, services, directives etc. without having to manually copy / paste a bunch of boilerplate code.

Additionally, you can more easily build and deploy your project using the CLI.

Should I use Angular or AngularJS?

With the performance advantages, Angular may seem like the best bet moving forward. There is a substantial learning curve to understanding the NodeJS / TypeScript ecosystem and one of the few advantages of AngularJS is that it just runs in the browser.

Using AngularJS makes sense if you have a small application and don't want to bother with learning the ins and outs of NodeJS and TypeScript.

There are also many existing projects out there that already use AngularJS and migrating to newer versions may not justify the cost of learning and rewriting code.

Outside of these cases, adopting "Angular" over the original "AngularJS" is preferred moving forward.

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Further reading about Angular

Angular 8 (formerly Angular 2) - The Complete Guide

Angular & NodeJS - The MEAN Stack Guide

The Complete Node.js Developer Course (3rd Edition)

The Web Developer Bootcamp

Best 50 Angular Interview Questions for Frontend Developers in 2019

How to build a CRUD Web App with Angular 8.0

React vs Angular: An In-depth Comparison

React vs Angular vs Vue.js by Example

Microfrontends — Connecting JavaScript frameworks together (React, Angular, Vue etc)

Building CRUD Mobile App using Ionic 4, Angular 8

How to Build Mobile Apps with Angular, Ionic 4, and Spring Boot

Ionic 4 & Angular Tutorial For Beginners - Crash Course