Angular For Beginners - Part Two - Modules, Components And Directives

In this article, you will learn about modules, components, and directives in Angular.

In the first part of the series, we discussed how to get started with Angular. Here is the link.

In this second part, we will discuss Modules, Components, and Directives in Angular. It is recommended that you go through the first part because the demo in this article is in continuation of that in the earlier part.

Modules

A module in Angular is a group of components, directives, services, etc. We will be having a look at each of these terms soon. Basically, think of a module as a group.

By default, we will have a file app.module.ts generated as below. It will be decorated with the directive @NgModule which tells Angular all the components, directives etc. that we will be using in our module.

App.module.ts file will list all the other modules and components that are imported. You will find that the first three lines import external modules. The Browser module is imported from the platform-browser package situated in the src folder. The second line imports the NgModule that is required because we are using the directive @NgModule. The third line imports the AppRoutingModule which we will discuss when we learn about routing.

These external modules that are imported are mentioned in the imports section.

The fourth import is AppComponent. This is a component that is present in our current module. Hence, it is mentioned in the declaration section.

You will find that underneath the providers, there is bootstrap mentioned. This specifies Angular to load AppComponent at startup.

Angular For Beginners - Modules, Components And Directives

Where is AppComponent declared? Go to the app.component.ts file and you will find it there.

Angular For Beginners - Modules, Components And Directives

Remember how we discussed Angular is a single page application? This page is index.html. If you navigate to the index.html, your default page, it will have <app-root></app-root>.

Angular For Beginners - Modules, Components And Directives

This app-root can be found below.

Angular For Beginners - Modules, Components And Directives

So basically, Angular knows that it has to show AppComponent on startup.

Before we move on to Components, can we create multiple modules in our application? Yes, sure we can. We will always have a root module which will be the starting module and the other modules are called as feature modules. To use them, you have to import them first in you root module.

When we have a huge application, it is a good idea to have multiple modules for implementing different functionalities. A shared module can be created for housing shared functionalities.

Components

Components are similar to a View we have in MVC architecture. It consists of the template (HTML) + class (properties,methods written in TypeScript) and metadata (some extra information).

Go to app.component.ts.

Angular For Beginners - Modules, Components And Directives

Here, you have an AppComponent class in TypeScript. You also have a templateUrl that specifies the HTML for the View and the rest is all metadata.

Let’s create a component Employee.

You can create it by writing the following command in the CLI. For this, create a new terminal.

ng generate component employee

Angular For Beginners - Modules, Components And Directives

This will create a new component "Employee". Angular will create four files for you as part of the component.

Angular For Beginners - Modules, Components And Directives

Also, when we had created the Employee component through Angular CLI, Angular registered our component class in the root module as below.

Angular For Beginners - Modules, Components And Directives

Directives

Directives help structure and modify your DOM. There are three types of directives in Angular.

Components

The templates in components that structure your DOM. We have already seen them.

Structural Directives

These are used to change the layout of the DOM. For example, we have ngIf and ngElse statements that help decide whether to display a particular element or not.

Now, let’s create an "employees" class as shown ahead. We don’t know its type, so we have defined it as an array of any type. We can create an "employees" class separately and could have defined employees as an array of that class. But for our demo purpose, we don’t need it.

  1. employees : any[] =[ {  
  2.     "employeeId" : 1,  
  3.     "employeeName" : "Tuba",  
  4.     "projectId":100  
  5.   },  
  6.   {   
  7.   "employeeId" : 2,  
  8.   "employeeName" : "Atul",  
  9.   "projectId":101  
  10.   },  
  11.   {   
  12.     "employeeId" : 3,  
  13.     "employeeName" : "Theran",  
  14.     "projectId":101  
  15.   }  
  16.   ]  
Angular For Beginners - Modules, Components And Directives
 
Before we display employees in our View, let's add bootstrap so, that we can get a responsive UI. This can be done easily by typing in the following command.

npm install bootstrap

Angular For Beginners - Modules, Components And Directives

Before we can use bootstrap classes in our employee.component.html, we need to import the styles in style.css as below.

  1. @import "~bootstrap/dist/css/bootstrap.min.css"  
Angular For Beginners - Modules, Components And Directives
 
Write the following code in employee.component.html.
  1. <div class="container">  
  2.   <h2>Employee List</h2>  
  3.   <div class="table-responsive">            
  4.   <table class="table" *ngIf = 'employees && employees.length' >  
  5.     <thead>  
  6.       <tr>          
  7.         <th>Employee Id</th>  
  8.         <th>Employee Name</th>  
  9.         <th>Project Id</th>         
  10.       </tr>  
  11.     </thead>  
  12.     <tbody>  
  13.       <tr *ngFor = 'let employee of employees'>  
  14.         <td>{{employee.employeeId}}</td>  
  15.         <td>{{employee.employeeName}}</td>  
  16.         <td>{{employee.projectId}}</td>  
  17.      </tr>  
  18.     </tbody>  
  19.   </table>  
  20.   <div>  
  21. </div>  
Angular For Beginners - Modules, Components And Directives
 
Here, we have used two structural directives - ngIf & ngFor. If employees class exists in the employee.component.ts file and its length is more than 0, then the table will be displayed. Also, we have used the ngFor structural directive to loop over the employees.

Structural directives always begin with an ‘*’.

We also need to add the employee component directive to app.component.html as below, so that it is displayed in the browser.

  1. <app-employee></app-employee>  
Angular For Beginners - Modules, Components And Directives
 
The directive ‘app-employee’ is mentioned in the metadata of employee.component.ts.
 
Angular For Beginners - Modules, Components And Directives

Now, let’s go to our browser. We can see a table displayed below.

Angular For Beginners - Modules, Components And Directives

Attribute Directives

These are directives that change the appearance or behavior of a DOM element. For example, ngStyle.

Let’s modify our code to use ngStyle as below.

  1. <td [ngStyle]="{'background-color':employee.projectId == '101' ? 'green' : 'red' }">{{employee.projectId}}</td>  
Angular For Beginners - Modules, Components And Directives
 
The above condition will apply a particular style after evaluating a particular expression. If the projectId is 101, the background for the column will be green, else red. Our browser will look like below.
 
Angular For Beginners - Modules, Components And Directives

This was all about directives. Let’s proceed to understand the different types of data bindings in Angular in the next part of this Angular Beginner’s course.

Happy learning!

This article was originally published on my website taagung.