Become a ninja with Angular (free sample)

The sixth version of the specification reached its final state in 2015. So it’s now supported by modern browsers, but there are still browsers in the wild that don’t support it yet, or only support it partially. And of course, now that we have a new specification every year (ES2016, ES2017, etc.), some browsers will always be late. You might be thinking: what’s the point of all this, if I need to be careful on what I can use? And you’d be right, because there aren’t that many apps that can afford to ignore older browsers. But, since virtually every JS developer who has tried ES2015+ wants to write ES2015+ apps, the community has found a solution: a transpiler.

A transpiler takes ES2015+ source code and generates ES5 code that can run in every browser. It even generates the source map files, which allows you to debug directly the ES2015+ source code from the browser. Back in 2015, there were two main alternatives to transpile ES2015+ code:

The source code of Angular itself was at first transpiled with Traceur, before switching to TypeScript. TypeScript is an open source language developed by Microsoft. It’s a typed superset of JavaScript that compiles to plain JavaScript, but we’ll dive into it very soon.

Let’s be honest: Babel has waaaay more steam than Traceur nowadays, so I would advise you to use it. It is now the de-facto standard in this area.

So if you want to play with ES2015+, or set it up in one of your projects, take a look at these transpilers, and add a build step to your process. It will take your ES2015+ source files and generate the equivalent ES5 code. It works very well but, of course, some of the new features are quite hard or impossible to transform in ES5, as they just did not exist. However, the current state is largely good enough for us to use without worrying, so let’s have a look at all these shiny new things we can do in JavaScript!

If you have been writing JS for some time, you know that the var declaration is tricky. In pretty much any other language, a variable is declared where the declaration is done. But in JS, there is a concept, called "hoisting", which actually declares a variable at the top of the function, even if you declared it later.

So declaring a variable like name in the if block:

is equivalent to declaring it at the top of the function:

ES2015 introduces a new keyword for variable declaration, let, behaving much more like what you would expect:

The variable name is now restricted to its block. let has been introduced to replace var in the long run, so you can pretty much drop the good old var keyword and start using let instead. The cool thing is, it should be painless to use let, and if you can’t, you have probably spotted something wrong with your code!

Since we are on the topic of new keywords and variables, there is another one that can be of interest. ES2015 introduces const to declare…​ constants! When you declare a variable with const, it has to be initialized and you can’t assign another value later.

As for variables declared with let, constants are not hoisted and are only declared at the block level.

One small thing might surprise you: you can initialize a constant with an object and later modify the object content.

But you can’t assign another object:

Same thing with arrays:

Not a new keyword, but it can also catch your attention when reading ES2015 code. There is now a shortcut for creating objects, when the object property you want to create has the same name as the variable used as the value.

Example:

can be simplified to:

Similarly, when you want to define a method in the object:

you can simplify it to:

This new feature can also catch your attention when reading ES2015 code. There is now a shortcut for assigning variables from objects or arrays.

In ES5:

Now, in ES2015, you can do:

And you will have the same result. It can be a little disturbing, as the key is the property to look for in the object and the value is the variable to assign. But it works great! Even better: if the variable you want to assign has the same name as the property, you can simply write:

The cool thing is that it also works with nested objects:

And the same is possible with arrays:

Of course it also works for arrays in arrays, or arrays in objects, etc.

One interesting use of this can be for multiple return values. Imagine a function randomPonyInRace that returns a pony and its position in a race.

The new destructuring feature assigns the position returned by the method to the position variable, and the pony to the pony variable! And if you don’t care about the position, you can write:

And you will only have the pony!

One of the characteristics of JavaScript is that it allows developers to call a function with any number of arguments:

The last case is the one that is the most relevant to us. Usually, we pass fewer arguments when the parameters are optional, like in the following example:

The optional parameters usually have a default value. The OR operator will return the right operand if the left one is undefined, as will be the case if the parameter was not provided (to be completely accurate, if it is falsy, i.e 0, false, "", etc.). Using this trick, the function getPonies can then be called:

This worked alright, but it was not really obvious that the parameters were optional ones with default values, without reading the function body. ES2015 introduces a more precise way to have default parameters, directly in the function definition:

Now it is perfectly clear that the size parameter will be 10 and the page parameter will be 1 if not provided.

The default value can also be a function call:

or even other variables, either global variables, or other parameters of the function:

This mechanism for parameters can also be applied to values, for example when using a destructuring assignment:

ES2015 introduces a new syntax to define variable parameters in functions. As said in the previous part, you could always pass extra arguments to a function and get them with the special arguments variable. So you could have done something like this:

But I think we can agree that it’s neither pretty nor obvious: since the ponies parameter is never used, how do we know that we can pass several ponies?

ES2015 gives us a way better syntax, using the rest operator …​:

ponies is now a true array on which we can iterate. The for …​ of loop used for iteration is also a new feature in ES2015. It makes sure that you iterate over the collection values, and not also over its properties as for …​ in would do. Don’t you think our code is prettier and more obvious now?

The rest operator can also work when destructuring data:

The rest operator is not to be confused with the spread operator which, I’ll give you that, looks awfully similar! But the spread operator is the opposite: it takes an array and spreads it in variable arguments. The only examples I have in mind are functions like min or max, that receive variable arguments, and that you might want to call on an array:

One of the most emblematic new features, and one that we will vastly use when writing an Angular app: ES2015 introduces classes to JavaScript! You can now easily use classes and inheritance in JavaScript. You always could, using prototypal inheritance, but that was not an easy task, especially for beginners.

Now it’s very easy, take a look:

Class declarations, unlike function declarations, are not hoisted, so you need to declare a class before using it. You may have noticed the special function constructor. It is the function being called when we create a new pony, with the new operator. Here it needs a color, and we create a new Pony instance with the color set to "blue". A class can also have methods, callable on an instance, as the method toString() here.

It can also have static attributes and methods:

Static methods can be called only on the class directly:

A class can have getters and setters, if you want to hook onto these operations:

And, of course, if you have classes, you also have inheritance out of the box in ES2015.

Animal is called the base class, and Pony the derived class. As you can see, the derived class has the methods of the base class. It can also override them:

As you can see, the keyword super allows calling the method of the base class, with super.speed() for example.

The super keyword can also be used in constructors, to call the base class constructor:

Promises are not so new, and you might know them or use them already, as they were a big part of AngularJS 1.x. But since you will use them a lot in Angular, and even if you’re just using JS, I think it’s important to make a stop.

Promises aim to simplify asynchronous programming. Our JS code is full of async stuff, like AJAX requests, and usually we use callbacks to handle the result and the error. But it can get messy, with callbacks inside callbacks, and it makes the code hard to read and to maintain. Promises are much nicer than callbacks, as they flatten the code, and thus make it easier to understand. Let’s consider a simple use case, where we need to fetch a user, then their rights, then update a menu when we have these.

With callbacks:

Now, let’s compare it with promises:

I like this version, because it executes as you read it: I want to fetch a user, then get their rights, then update the menu.

As you can see, a promise is a 'thenable' object, which simply means it has a then method. This method takes two arguments: one success callback and one reject callback. The promise has three states:

When the promise is fulfilled, then the success callback is called, with the result as an argument. If the promise is rejected, then the reject callback is called, with a rejected value or an error as the argument.

So, how do you create a promise? Pretty simple, there is a new class called Promise, whose constructor expects a function with two parameters, resolve and reject.

Once you have created the promise, you can register callbacks, using the then method. This method can receive two parameters, the two callbacks you want to call in case of success or in case of failure. Here we only pass a success callback, ignoring the potential error:

Once the promise is resolved, the success callback (here simply logging the user on the console) will be called.

The cool part is that it flattens the code. For example, if your resolve callback is also returning a promise, you can write:

but more beautifully:

Another interesting thing is the error handling, as you can use one handler per promise, or one for all the chain.

One per promise:

One for the chain:

You should seriously look into Promises, because they are going to be the new way to write APIs, and every library will use them. Even the standard ones: the new Fetch API does for example.

One thing I like a lot in ES2015 is the new arrow function syntax, using the 'fat arrow' operator (⇒). It is SO useful for callbacks and anonymous functions!

Let’s take our previous example with promises:

can be written with arrow functions like this:

How cool is it? THAT cool!

Note that the return is also implicit if there is no block: no need to write user ⇒ return getRights(user). But if we did have a block, we would need the explicit return:

And it has a special trick, a great power over normal functions: the this stays lexically bounded, which means that these functions don’t have a new this as other functions do. Let’s take an example, where you are iterating over an array with the map function to find the max.

In ES5:

You would expect this to work, but it doesn’t. If you have a good eye, you may have noticed that the forEach in the find function uses this, but the this is not bound to an object. So this.max is not the max of the maxFinder object…​ Of course you can fix it easily, using an alias:

or binding the this:

or even passing it as a second parameter of the forEach function (as it was designed for):

But there is now an even more elegant solution with the arrow function syntax:

That makes the arrow functions the perfect candidates for anonymous functions in callbacks!

We were talking about promises earlier, and it’s worth knowing that another keyword was introduced to handle them more synchronously: await.

This is not a feature introduced in ECMAScript 2015 but in ECMAScript 2017, and to use await, your function must be marked as async. When you use the await keyword in front of a Promise, you pause the execution of your async function, wait for the Promise to resolve, and then resume the execution of the async function. The returned value will be the resolved value.

So we can write our previous example using async/await like this:

And your code now looks like it is synchronous! Another cool feature of async/await is that you can use a simple try/catch to handle errors:

Note that async/await is still asynchronous, although it looks like synchronous. The function execution is paused and resumed, but just like with callbacks, this doesn’t block the thread: other JavaScript events can be handled while the execution is paused.

This is a short one: you now have proper collections in ES2015. Yay \o/! We used to have dictionaries filling the role of a map, but we can now use the class Map:

We also have a class Set:

You can iterate over a collection, with the new syntax for …​ of:

You’ll see that the for …​ of syntax is the one the Angular team chose in order to iterate over a collection in a template.

Composing strings has always been painful in JavaScript, as we usually have to use concatenation:

Template literals are a new small feature, where you have to use backticks (`) instead of quotes or simple quotes, and you have a basic templating system, with multiline support:

The multiline support is especially great when your are writing HTML strings, as we will do for our Angular components:

One last feature is the ability to tag them. You can define a function, and apply it to a template string. Here askQuestion adds an interrogation point at the end of the string:

So what’s the difference with a simple function? The tag function in fact receives several arguments:

For example if we have a template string containing expressions:

then the tag function will receive the various static and dynamic parts. Here we have a tag function to uppercase the names of the protagonists:

Let’s now talk about one of the big changes introduced: modules.

A standard way to organize functions in namespaces and to dynamically load code in JS has always been lacking. Node.js has been one of the leaders in this, with a thriving ecosystem of modules using the CommonJS convention. On the browser side, there is also the AMD (Asynchronous Module Definition) API, used by RequireJS. But none of these were a real standard, thus leading to endless discussions on what’s best.

ES2015 aims to create a syntax using the best from both worlds, without caring about the actual implementation. The Ecma TC39 committee (which is responsible for evolving ES2015 and authoring the specification of the language) wanted to have a nice and easy syntax (that’s arguably CommonJS’s strong suit), but to support asynchronous loading (like AMD), and a few goodies like the possibility to statically analyze the code by tools and support cyclic dependencies nicely. The new syntax handles how you export and import things to and from modules.

This module thing is really important in Angular, as pretty much everything is defined in modules, which you have to import when you want to use them. Let’s say I want to expose a function to bet on a specific pony in a race and a function to start the race.

In races.service.js:

As you can see, this is fairly easy: the new keyword export does a straightforward job and exports the two functions.

Now, let’s say one of our application components needs to call these functions.

In another file:

That’s what is called a named export. Here we are importing the two functions, and we have to specify the filename containing these functions - here 'races.service'. Of course, you can import only one method if you need, and you can even give it an alias:

And if you want to use all the exported symbols (functions, constants, classes etc.) from the module, you can use a wildcard '*'.

As you would do with other languages, use the wildcard with care, only when you really want all the functions, or most of them. As this will be analyzed by our IDEs, we will see auto-import soon and that will free us from the bother of importing the right things.

With a wildcard, you have to use an alias, and I kind of like it, because it makes the rest of the code clearer:

If your module exposes only one function or value or class, you don’t have to use named export, and you can leverage the default keyword. It works great for classes for example:

Notice the lack of curly braces to import a default. You can import it with the alias you want, but to be consistent, it’s better to call the import with the module name (except if you have multiple modules with the same name of course, then you can choose an alias that allows you to distinguish them). And of course, you can mix default export with named ones, but obviously with only one default per module.

In Angular, you’re going to use a lot of these imports in your app. Each component and service will be a class, generally isolated in their own file and exported, and then imported when needed in other components.

That ends our gentle introduction to ES2015+. We skipped some other parts, but if you’re comfortable with this chapter, you will have no problem writing your apps in ES2015+. If you want to have a deeper understanding of this, I highly recommend Exploring JS by Axel Rauschmayer or Understanding ES6 from Nicholas C. Zakas…​ Both ebooks can be read online for free, but don’t forget to buy it to support their authors. They have done great work! Actually I’ve re-read Speaking JS, Axel’s previous book, and I again learned a few things, so if you want to refresh your JS skills, I definitely recommend it!

You may have heard that Angular apps can be written in TypeScript. And you may be wondering what TypeScript is, or what it brings to the table.

JavaScript is dynamically typed. That means you can do things like:

And it works. That’s great for all sort of things, as you can pass pretty much any object to a function and it works, as long as the object has the properties the function needs:

This dynamic nature allows wonderful things but it is also a pain for a few other reasons compared to more statically-typed languages. The most obvious might be when you call an unknown function in JS from another API, you pretty much have to read the doc (or, worse, the function code) to know what the parameter should look like. Take a look at our previous example: the method printColor needs a parameter with a color property. That can be hard to guess, and of course it is much worse in day-to-day work, where we use various libraries and services developed by fellow developers. One of Ninja Squad’s co-founders is often complaining about the lack of types in JS, and finds it regrettable he can’t be as productive and write as good code as he would in a more statically-typed environment. And he is not entirely wrong, even if he is sometimes ranting for the sake of it too! Without type information, IDEs have no real clue if you’re doing something wrong, and tools can’t help you find bugs in your code. Of course, we have tests in our apps, and Angular has always been keen on making testing easy, but it’s nearly impossible to have a perfect test coverage.

That leads to the maintainability topic. JS code can become hard to maintain, despite tests and documentation. Refactoring a huge JS app is no easy task, compared to what could be done in other statically-typed languages. Maintainability is a very important topic, and types help humans and tools to avoid mistakes when writing and maintaining code. Google has always been keen to push new solutions in that direction: it’s easy to understand as they have some of the biggest web apps of the world, with GMail, Google apps, Maps…​ So they have tried several approaches to front-end maintainability: GWT, Google Closure, Dart…​ All trying to help writing big webapps.

For Angular, the Google team wanted to help us to write better JS, by adding some type information to our code. It’s not a very new concept in JS. It was even the subject of the ECMAScript 4 specification, which was later abandoned. At first they announced AtScript, as a superset of ES2015+ with annotations (types annotations and another kind I’ll discuss later). They also announced the support of TypeScript, the Microsoft language, with additional type annotations. And then, a few months later, the TypeScript team announced that they had worked closely with the Google team, and the new version of the language (1.5) would have all the shiny new things AtScript had. And the Google team announced that AtScript was officially dropped, and that TypeScript was the new top-notch way to write Angular apps!

I think this was a smart move for several reasons. For one, no one really wants to learn another language extension. And TypeScript was already there, with an active community and ecosystem. I never really used it before Angular, but I heard good things about it, from various people. TypeScript is a Microsoft project. But it’s not the Microsoft you have in mind, from the Ballmer and Gates years. It’s the Microsoft of the Nadella era, the one opening up to its community, and, well, open-source. Google knows this, and it’s far better for them to contribute to an existing project, rather than to have to bear the burden of maintaining their own. And the TypeScript framework will gain a huge popularity boost: win-win, as your manager would say.

But the main reason to bet on TypeScript is the type system it offers. It’s an optional type system that helps without getting in the way. In fact, after coding some time with it, you’ll probably want to code every application with it. I do like what they have done, and we will have a look at what TypeScript offers in the next section. At the end, you’ll have enough understanding to read any Angular code, and you’ll be able to choose whether you want to use it or not, in your apps.

You may be wondering: why use typed code in Angular apps? Let’s take an example. Angular 1 and 2 have been built around a powerful concept named "dependency injection". You might already be familiar with it, as it is a common design pattern used in several frameworks for different languages and, as I said, already used in AngularJS 1.x.

To sum up what dependency injection is, think about a component of the app, let’s say RaceList, needing to access the races list that the service RaceService can give. You would write RaceList like this:

But it has several flaws. One of them is the testability: it is now very hard to replace the raceService by a fake (mock) one, to test our component.

If we use the Dependency Injection (DI) pattern, we delegate the creation of the RaceService to the framework, and we simply ask for an instance. The framework is now in charge of the creation of the dependency, and, well, injects it:

Now, when we test this class, we can easily pass a fake service to the constructor:

But how does the framework know what to inject in the constructor? Good question! AngularJS 1.x relied on the parameter’s names, but it had a severe limitation, because minification of your code would have changed the param name…​ You could use the array syntax to fix this, or add a metadata to the class:

We had to add some metadata for the framework to understand what classes needed to be injected with. And that’s exactly what type annotations give: a metadata giving the framework a hint it needs to do the right injection. In Angular, using TypeScript, we can write our RaceList component like:

Now the injection can be done!

That’s why we’re going to spend some time learning TypeScript (TS). Angular is clearly built to leverage this language, so we will have the easiest time writing our apps using it. And the Angular team really hopes to submit the type system to the standard committee, so maybe one day we’ll have types in JS, and all this will be usual.

Let’s dive in!

The general syntax to add type info in TypeScript is rather straightforward:

The types are easy to remember:

In such cases, the types are optional because the TS compiler can guess them (it’s called "type inference") from the values.

The type can also come from your app, as with the following class Pony:

TypeScript also supports what some languages call "generics", for example for an array:

The array can only contain ponies, and the generic notation, using <>, indicates this. You may be wondering what the point of doing this is. Adding types information will help the compiler catch possible mistakes:

So, if you need a variable to have multiple types, does it mean you’re screwed? No, because TS has a special type, called any.

It’s really useful when you don’t know the type of a value, either because it’s from a dynamic content or from a library you’re using.

If your variable can only be of type number or boolean, you can use a union type:

TypeScript also offers enum. For example, a race in our app can be either ready, started or done.

The enum is in fact a numeric value, starting at 0. You can set the value you want, though:

Since TypeScript 2.4, you can even specify a string value:

To be honest though, we don’t use enums a lot in our projects: we use union types. They are simpler and cover roughly the same use-cases:

TypeScript even allows you to create your own types, so you could do something like:

You can also set the return type of a function:

If the function returns nothing, you can show it using void:

That’s a good first step. But as I said earlier, JavaScript is great for its dynamic nature. A function will work if it receives an object with the correct property:

This function can be applied to any object with a score property. How do you translate this in TypeScript? It’s easy: you define an interface, which is like the "shape" of the object.

It means that the parameter must have a property called score of the type number. You can name these interfaces, of course:

You’ll see that we often use interfaces throughout the book to represent our entities. We use interfaces for our models in our other projects as well. We usually append a Model suffix to make it clear. It’s then very easy to create a new entity:

Another treat of JavaScript is that arguments are optional. You can omit them, and they will become undefined. But if you define a function with typed parameter in TypeScript, the compiler will shout at you if you forget them:

To show that a parameter is optional in a function (or a property in an interface), you can add ? after the parameter. Here, the points parameter could be optional:

You may also be interested in describing a parameter that must have a specific function instead of a property. The interface definition will be:

A class can implement an interface. For us, the Pony class should be able to run, so we can write:

The compiler will force us to implement a run method in the class. If we implement it badly, by expecting a string instead of a number for example, the compiler will yell:

You can also implement several interfaces if you want:

And an interface can extend one or several others:

When you’re defining a class in TypeScript, you can have properties and methods in your class. You may realize that properties in classes are not a standard ES2015+ feature. It is only possible in TypeScript.

Everything is public by default, but you can use the private keyword to hide a property or a method. If you add private or public to a constructor parameter, it is a shortcut to create and initialize a private or public member:

Which is the same as the more verbose:

These shortcuts are really useful and we’ll rely on them a lot in Angular!

When working with external libraries written in JS, you may think we are doomed because we don’t know what types of parameter the function in that library will expect. That’s one of the cool things with the TypeScript community: its members have defined interfaces for the types and functions exposed by the popular JavaScript libraries!

The files containing these interfaces have a special .d.ts extension. They contain a list of the library’s public functions. A good place to look for these files is DefinitelyTyped. For example, if you want to use TS in your AngularJS 1.x apps, you can download the proper file from the repo directly with NPM:

or download it manually. Then include the file at the top of your code, and enjoy the compilation checks:

/// <reference path="angular.d.ts" /> is a special comment recognized by TS, telling the compiler to look for the interface angular.d.ts. Now, if you misuse an AngularJS method, the compiler will complain, and you can fix it on the spot, without having to manually run your app!

Even cooler, since TypeScript 1.6, the compiler will auto-discover the type definitions of an NPM library if they are packaged with the library itself. More and more projects are adopting this approach, and so is Angular. So you don’t even have to worry about including the interfaces in your Angular project: the TS compiler will figure it out by itself if you are using NPM to manage your dependencies!

This feature was added in TypeScript 1.5, notably to help support Angular. Indeed, as we will shortly see, Angular components can be described using decorators. You may not have heard about decorators, as not every language has them. A decorator is a way to do some meta-programming. They are fairly similar to annotations which are mainly used in Java, C# and Python, and maybe other languages I don’t know. Depending on the language, you add an annotation to a method, an attribute, or a class. Generally, annotations are not really used by the language itself, but mainly by frameworks and libraries.

Decorators are really powerful: they can modify their target (method, classes, etc.), and for example alter the parameters of the call, tamper with the result, call other methods when the target is called or add metadata for a framework (which is what Angular decorators do). Until now, it was not something that was possible in JavaScript. But the language is evolving and there is now an official proposal for decorators, which may be standardized one day in the future. Note that the TypeScript implementation goes slightly further than the proposed standard.

In Angular, we will use the decorators provided by the framework. Their role is fairly basic: they add some metadata to our classes, attributes or parameters to say things like "this class is a component", "this is an optional dependency", "this is a custom property", etc. You are not required to use them, as you can add the metadata manually (if you want to stick to ES5 for example), but the code will definitely be more elegant using decorators, as provided by TypeScript.

In TypeScript, decorators start with an @, and can be applied to a class, a class property, a function or a function parameter. They can’t be applied to a constructor, but can be applied to its parameters.

To have a better grasp on this, let’s try to build a simple decorator, @Log(), that will log something every time a method is called.

It will be used like this:

To define it, we have to write a method returning a function like this:

Depending on what you want to apply your decorator to, the function will not have exactly the same arguments. Here we have a method decorator that takes 3 parameters:

Here we simply log the method name, but you could do pretty much whatever you want: interfere with the parameters, the result, calling another function, etc.

So, in our simple example, every time the getRace() or getRaces() methods are called, we’ll see a trace in the browser logs:

As a user, let’s look at what a decorator in Angular looks like:

The @Component decorator is added to the class Home. When Angular loads our app, it will find the class Home and will understand that it is a component, based on the metadata the decorator will add. Cool, huh? As you can see, a decorator can also receive parameters, here a configuration object.

I just wanted to introduce the raw concept of decorators; we’ll look into every decorator available in Angular throughout the book.

So my advice would be to give TypeScript a try! All my examples from here will be in TypeScript, as Angular and all the tooling around are really designed for it.

You can use the readonly keyword to mark the property of a class or interface as…​ read only! That way, the compiler will refuse to compile any code trying to assign a new value to the property:

The keyof keyword can be used to get a type representing the union of the names of the properties of another type. For example, you have a PonyModel interface:

You want to build a function that returns the value of a property. You could implement a naive version:

Two problems here:

This is where keyof can shine. keyof allows you to list all the keys of a type:

So we can use this type to make getProperty safer, by declaring that:

We killed two birds with one stone here:

Now let’s see how we can leverage keyof to do even more.

Let’s say you want to create a type that has exactly the same properties as PonyModel, but you want every property to be optional. You can of course define it manually:

But you can do something more generic with a mapped type:

The Partial type is a transformation that applies the ? modifier to every property of a type! In fact, you don’t have to define the type Partial yourself, because since version 2.1, it’s part of the language itself, and it’s declared exactly like in the above example.

TypeScript offers other mapped types out of the box.

Union types are really handy. Let’s say your application has authenticated users and anonymous users, and sometimes you need to do a different action depending on that. You can model this as:

I don’t know about you, but I don’t like these as …​ explicit casts. Maybe we can do better?

One possibility is to use a type guard, a special function whose sole purpose is to help the TypeScript compiler.

This is better! But we still need to return a default value, even if we covered all the possibilities.

We can slightly improve the situation if we drop the type guards and use a union type instead.

This is even better, as TypeScript automatically narrows the type in the else branch.

Sometimes you know that the model will grow in the future, and that more cases will need to be handled. For example if you introduce an AdminUser. In that case, you can use a switch. A switch statement will break if one of the cases is not handled. So introducing our AdminUser, or another type of user later, would automatically add compilation errors in every place you need to handle it!

I hope these patterns will help you. Now let’s focus on Web Components.

Components are an old fantasy in development. Something you can grab off the shelves and drop into your app, something that would work right away and bring a needed functionality to your users.

My friends, this time has come.

Well, maybe. At least, there is the start of something.

That’s not completely new. We have had components in web development for quite some time, but they usually require some kind of dependency, like jQuery, Dojo, Prototype, AngularJS, etc. Not necessarily libraries you wanted to add to your app.

Web Components attempt to solve this problem: let’s have reusable and encapsulated components.

They rely on a set of emerging standards that browsers don’t perfectly support yet. But, still, it’s an interesting topic, even if there’s a chance we’ll have to wait a few years to use them fully, or even if the concept never takes off.

This emerging standard is defined in 3 specifications:

Note that the samples are most likely to work in a recent Chrome or Firefox browser.

Custom elements are a new standard allowing developers to create their own DOM elements, making something like <ns-pony></ns-pony> a perfectly valid HTML element. The specification defines how to declare such elements, how to make them extend existing elements, how to define your API, etc.

Declaring a custom element is done using customElements.define:

And you can then use it:

Note that the name must contain a dash, so that the browser knows it is a custom element. Of course, your custom element can have properties and methods, and it also has lifecycle callbacks, to be able to execute code when the component is inserted or removed, or when one of its attributes changes. It can also have a template of its own. Maybe the ns-pony displays an image of the pony or just its name:

If you try to look at the DOM, you’ll see <ns-pony><h1>General Soda</h1></ns-pony>. But that means the CSS and JavaScript logic of your app can have undesired effects on your component. So, usually, the template is hidden and encapsulated in something called Shadow DOM, and you’ll only see <ns-pony></ns-pony> if you inspect the DOM, despite the fact that the browser displays the pony’s name.

With a mysterious name like this, you expect something with great powers. And surely it is. The Shadow DOM is a way to encapsulate the DOM of our component. This encapsulation means that the stylesheet and JavaScript logic of your app will not apply on the component and ruin it inadvertently. It gives us the perfect tool to hide the internals of a component, and be sure nothing leaks from the component to the app, or vice-versa.

Going back to our previous example:

If you try to inspect it now you should see:

Now, even if you try to add some style to the h1 elements, the visual aspect of the component won’t change at all: that’s because the Shadow DOM acts like a barrier.

Until now, we just used a string as a template of our web component. But that’s usually not the way you do that. Instead, the best practice is to use the <template> element.

A template specified in a <template> element is not displayed in your browser. Its main goal is to be cloned in an element at some point. What you declare inside will be inert: scripts don’t run, images don’t load, etc. Its content can’t be queried by the rest of the page using usual methods like getElementById() and it can be safely placed anywhere in your page.

To use a template, it needs to be cloned:

All these things put together make the Web Components. I’m far from being an expert on this topic, and there are all sorts of twisted traps on this road.

As Web Components are not fully supported by every browser, there is a polyfill you can include in your app to make sure it will work. The polyfill is called web-component.js, and it’s worth noting that it is a joint effort from Google, Mozilla and Microsoft among others.

On top of this polyfill, a few libraries have seen the light. All aim to facilitate working with Web Components, and often come with some ready-to-use Web Components.

Among the most notable initiatives, you find:

I won’t go into the details, but you can easily use an already existing component. Let’s say you want a Google Map in your app:

There are a LOT of components out there. You can have an overview on https://www.webcomponents.org/.

You can do a lot of cool things with LitElement and other similar frameworks, like two-way data binding, default values for attributes, emit custom events, react to attribute changes, repeat elements if we give a collection to a component, etc.

That’s obviously far too short a chapter to tell you everything there is to say on Web Components, but you’ll see that some of the concepts are going to pop out along your read. And you’ll definitely see that the Google team designed Angular to make it easy to use Web Components with our Angular components. It is even possible to export our own Angular components as Web Components, with the help of Angular Elements.

Pretty much all the modern JavaScript tools are built for Node.js and NPM these days. You’ll have to install Node.js and NPM on your system. The best way to do that depends on your operating system - you can find more information on the official website. Make sure you have a recent enough version of Node.js (by executing node --version).

You could setup everything by yourself, starting with a TypeScript project, then install every dependency needed, etc.

But in a real project, you’ll probably have to set up several other things too, like:

But it’s a bit cumbersome to setup everything yourself, especially when there are sooooo many tools to learn first.

These past few years, a lot of small project generators have seen the light, pretty much all using the great Yeoman. It used to be the case for AngularJS 1.x, and there were a few attempts for Angular from the community.

But this time, the Google team has been working on this issue, and they have come up with something: Angular CLI.

Angular CLI is a command line utility to easily quick start a project, already configured with Webpack as a build tool (the popular kid these years), tests, packaging, etc.

The idea is not new, and is in fact borrowed from another popular framework: EmberJS and its popularly acclaimed ember-cli.

The tool is under continuous development, with a dedicated Google team working on it and making it better and better. It is now the recommended and de facto standard way of creating and building Angular apps. So let’s give it a try, and discover the ton of cool stuff packed into it!

First let’s install Angular CLI, and generate a new application with the ng new command. If you want to use exactly the same CLI version than we are (19.0.6), you can use npm install -g @angular/cli@19.0.6 instead.

This will create a project skeleton in a new directory called ponyracer. From this directory, you can start your app with:

This will start a small HTTP server locally, with a hot reload configuration. It means that every time you modify and save a file, the server will rebuild the app, and the browser will reload it immediately.

Tada! You have your first application up and running! 🎉

Let’s dive for a few seconds into the generated code.

Open the project in your preferred IDE. You can use pretty much anything you want, but you should activate the TypeScript support for maximum comfort. Pick your favorite: Webstorm, Visual Studio Code…​ All of them have great support for TypeScript.

You should see a bunch of configuration files in the root directory: welcome to Modern JavaScript!

The first one you may recognize is the package.json file: that’s where the dependencies of the application are defined. You can have a look inside, it should now contain the following dependencies:

TypeScript itself has a configuration file tsconfig.json (and another one called tsconfig.app.json), which stores the compilation options. As we saw in the previous chapters, we are using TypeScript with decorators (hence the two options about decorators). The sourceMap option allows you to generate source maps, i.e. files that contain a mapping between the generated JavaScript code and the original TypeScript code. Those source maps are used by the browser to let you debug the JavaScript code it executes by stepping through the original TypeScript code that you have written.

TypeScript projects often also use ESLint (that we need to add to a project, as we show in the exercise Getting Started of our online course), a linter used to check your code against the best practices. ESLint has its own options, stored in .eslintrc.json, where you add/remove some of its rules.

Angular CLI itself has a configuration file angular.json if you want to override some of its defaults.

Now that we have been over the configuration, let’s see the application code.

As we saw in the previous section, a component is a combination of a view (the template) and some logic (our TS class). The CLI has already created one for us: src/app/app.component.ts. Let’s check it out:

Our application itself is a simple component. To tell Angular that it is a component, we use the @Component decorator. And to be able to use this decorator, we have to import it as you can see at the top of the file.

When you write new components, don’t forget to import the Component decorator. You may forget to do so at the beginning, but it won’t last, as the compiler will yell at you! ;)

You’ll see that most of the things we need are in the @angular/core module, but that’s not always the case. For example, when dealing with HTTP, we’ll use imports from @angular/common/http; or, if we use the router, we’ll import from @angular/router, etc.

The @Component decorator is expecting a configuration object. We’ll see later in detail what you can configure here, but for now let’s start with the selector property. It will tell Angular what to look for in our HTML pages. Every time Angular finds an element in our HTML which matches the selector of the component, it will create an instance of the component, and replace the content of the element by the template of the component.

So, here, every time our HTML contains an element like <ns-root />, Angular will instantiate a new instance of our AppComponent class.

A component must also have a template. We can have an inline template or externalize it in another file, like the CLI does:

The corresponding HTML is defined in app.component.html, with a bunch of static elements, except the first h1:

Finally, you can see that the component has an additional option: imports.

Our component is standalone (the standalone: true property is by default since Angular v19). This means that the component doesn’t need to be declared in an Angular module in order to be usable.

The property imports: [] is not always strictly required. Its goal is to tell Angular which other components, pipes and directives can be used inside the template of our component. Most of the components that we create use pipes and directives provided by Angular. These very commonly used pipes and directives are declared in an Angular module named CommonModule. Using imports: [CommonModule] in the configuration of our component thus makes it possible to use them all in its template, or you can explicitly import only the ones you need.

We were talking about ES2015+ and TS modules in the first chapters, which define imports and exports. The TypeScript compiler, when it sees an interface such as Race being used in the TypeScript source code, must know where to find the definition of this interface Race. That’s why you need to import it.

The imports property of the @Component decorator serves a similar purpose: if Angular were to find an element <ns-race> in the template of AppComponent, it would need to know where and how the corresponding RaceComponent is defined. To let it know where it can find its definition, you would need to add the RaceComponent to the imports of the AppComponent decorator.

Finally, we need to start our app, using the bootstrapApplication function. Angular CLI will by default generate a separate file containing this bootstrap logic: main.ts:

As you can see, what it expects is the root component of the application: AppComponent.

Yay! But wait a second. We need an HTML file to serve to our users, right?

The CLI created an index.html file for us, which is the single page of our application. You might wonder how it could possibly work, since it doesn’t contain any script element.

When you run ng serve, the CLI calls the TypeScript compiler. The compiler outputs JavaScript files. The CLI then bundles them and adds the necessary script elements to the index.html file (using Vite behind the scenes).

Hopefully, you now have a better understanding of the various parts of this first Angular application. It’s not really a dynamic app yet, and we could have done the same in one second in a static HTML page, I’ll give you that. So let’s jump to the next sections, and learn all about signals and templating.

Signals basics

Reactive Programming

Building components and directives

Reacting to signal changes

Send and receive data with Http

Advanced components and directives

Signals: advanced topics

Performances

The templating syntax

Building components and directives

Performances

The templating syntax

Building components and directives

Forms

Send and receive data with Http

Internationalization

Signals

Advanced observables

Signals

Deferred loading with @defer

Signals

Styling components and encapsulation

Signals

Control flow syntax

Deferred loading with @defer

Building components and directives

Building components and directives

Router

Signals

Advanced observables

Router

Dependency Injection

Router

Standalone components

Going to production

Dependency Injection

Router

Standalone components

Going to production

Standalone components

Performances

Router

Forms

Standalone components

Performances

Forms

Advanced components and directives

Going to production

The templating syntax

Going to production

Global

Reactive Programming

Global

From zero to something

Testing your app

Send and receive data with Http

Going to production

Internationalization

Internationalization

Testing your app

Internationalization

Global

The wonderful world of Web Components

Reactive Programming

Global

From zero to something

Global

A gentle introduction to ECMAScript 2015+

Diving into TypeScript

Advanced TypeScript

From zero to something

Testing your app

Internationalization

Going to production

Global

From zero to something

Testing your app

Global

The wonderful world of Web Components

From zero to something

Global

A gentle introduction to ECMAScript 2015+

From zero to something

Testing your app

Forms

Router

Angular compiler

Advanced components and directives

Going to production

Global

From zero to something

Global

From zero to something

Router

Global

From zero to something

Performances

Going to production

Global

From zero to something

Pipes

Styling components and encapsulation

Send and receive data with Http

Router

Advanced observables

Internationalization

Global

The wonderful world of Web Components

From zero to something

Dependency Injection

Reactive Programming

Services

Testing your app

Advanced components and directives

Advanced observables

Global

Building components and directives

Forms

Send and receive data with Http

Global

Pipes

Forms

Send and receive data with Http

Router

Internationalization

Global

Forms

Send and receive data with Http

Router

Advanced components and directives

Global

Forms

Router

Angular compiler

Global

The templating syntax

Dependency Injection

Pipes

Services

Testing your app

Forms

Send and receive data with Http

Router

Advanced observables

Internationalization

Global

Forms

Global

Testing your app

Forms

Router

Advanced observables

Global

From zero to something

Pipes

Global

From zero to something

The templating syntax

Dependency Injection

Pipes

Styling components and encapsulation

Services

Testing your app

Forms

Send and receive data with Http

Router

Changelog

Global

Diving into TypeScript

The templating syntax

Dependency Injection

Pipes

Reactive Programming

Building components and directives

Testing your app

Forms

Send and receive data with Http

Router

Zones and the Angular magic

Global