Introduction to Modules in Angular

Overview

Angular Framework is strictly modularized by design, you don't have to explicitly define your own standards to segregate the Angular Code. Angular has its own thing for defining modules, which is NgModule. NgModule is the most important part of the Angular application. They help to create separate modules, these modules can be loaded on demand by the technique of router lazy loading. We will look at this lazy loading technique in the next chapter.

Introduction

Every Angular application has at least contains one NgModule class, the root module in angular known as AppModule, and resides in app.module.ts. The application launched by bootstrapping the root NgModule(AppModule). Most applications would have much more NgModule aka feature modules. The root NgModule includes all child NgModules in a hierarchy of any depth.

A Basic Use of Modules

An NgModule is a class marked by the @NgModule decorator
NgModule is a collection of directives, components, pipes, services, other dependency modules, providers, etc.
It creates a logical boundary for dependencies resolution.
It helps to organize related things together.
It can control what can be exposed from @NgModule.

NgModule Metadata

You can pass configuration in metadata@NgModule decorator

option	Description
declarations	Things used on HTML belonging to this NgModule like components, directives, and pipes
exports	The subset of declarations that should be reusable in the component templates of other NgModules
imports	Other modules whose exported (components, directives, pipes) classes and services are needed in this NgModule.
providers	Creators of services that this NgModule contributes to the global collection of services
bootstrap	The main application view (entry point), called a root component, hosts all other application views. Only the root NgModule should set the bootstrap property

Declarations Array

declarations array consist of classes that are used on HTML from the NgModule
- Component
- Directive
- Pipe
Without declaring these classes inside the declarations array, they can not be used on HTML.

Providers Array

It contains
- Configurable dependencies
- Providers / Services

Imports Array

Imports other @angular NgModule where the current module is dependent on
From imported NgModule we can use exported classes on current module component templates
Providers (services) will also be available for injection and use.
To use 3rd party library features, include respective NgModule in the desired module, wherever you wanted to use library features.

Exports Array

export NgModules, components, pipes, and directives, that can be used by importer NgModule.
To export components/pipes/directives, they should also be a part of the declarations array.
If we don't export the aforementioned classes, you would see an error in the console for eg. 'my-component' is not a known element

Bootstrap

Mention which component should be the root of an application.
Ideally, there should be a single root component per application.
But there could be cases, that you may add multiple root components in the bootstrap option.

NgModules and Components

NgModules	Components
It collates different building blocks of an application together	Component consists of the template and its associated class
It has providers array to resolve dependencies	Component instance has change detector for resolving dependencies
It forms injector tree for lazily loaded module	Components create view DOM tree for nested components
NgModule providers are lookup after components providers	Change detector resolve dependency on component level, then NgModule level

NgModules and JavaScript Modules

NgModules	Javascript Modules
Angular specific construct	JS modules are part of the browser itself
It combines different pieces of Angular Framework under a single block	It can import or export function/variables/classes
It helps to divide an application into small blocks	ES module help to modularize code
Encapsulates code to hide implementation details	helps to create a boundaries between different parts of an application
Accepts metadata configuration	It does not accept any metadata configuration object
helps to keep the Separation of concerns	Avoid leaking code to the global namespace

Angular Module Example

Below is an example of a Simple NgModule

Create the Application

You can run below command in command line / terminal. This helps to create a brand new angular application.

ng new - helps to create a new angular project
employee-management - project and folder name
--prefix=em - is selector prefix used when new components/directives are generated.

Routing Module

When creating an app, it would ask to enable routing, just say "YES" there.
It automatically adds AppRoutingModule
It would have empty routes registered with RouterModule's forRoot method.
Add fallback route to routes array using wildcard (**) check

Root Module

AppRoutingModule is already imported in AppModule imports arrray

Creating a Module

Again, you can use CLI for generating NgModule.

Generate NgModule Using CLI

It generates HomeModule inside home folder

home.module.ts

Components

Create Home Component

ng g c home - Create a HomeComponent
--module=home.module - Declare HomeComponent under declartions of HomeModule

Folder Structure

You can additionally add routing of HomeModule, register home path against HomeComponent with the help of RouterModule's forChild method.

home.module.ts

Using HomeModule in AppModule

Import HomeModule inside a Root Module i.e. AppModule
This way we extended the RootModule with the feature NgModule.

Create NgModule and Component along with Routing Using CLI

We've already said this multiple times, Angular CLI is helping developers by automating most of the stuff. There should be something to simplify these many file creation. Yo! that's possible, you would just need to fire the below command to generate the scaffold files.

Let's try to understand the above command in parts

ng generate module home - Generate home.module.ts
--route home - Create a HomeComponent
--route home - Create route configuration for the HomeComponent, and mentions it in imports of a home.module.ts
--module app.module - Include the HomeModule inside AppModule.

Changes in app.component.html

Add placeholder for router, to keep watch on URL and render a matched view inside it.

Angular Libraries

Module	Description
CommonModule / BrowserModule	Most of the common components, directives, services, and pipes
RouterModule	Routing related Component, directives and services are imported from here
HttpClientModule	API for making HTTP calls
FormsModule	For template-driven form
ReactiveFormsModule	For model driven form
ElementModule	use methods to create web components from it
i18nModule	Use for i18n or localization

Bootstrapping an Application

Application has to be bootstrap by calling bootstrapModule method, which accepts the root module in angular (AppModule) from main.ts.

Conclusion

What is module in angular?
How to create a NgModule?
How feature module created and imported?
Difference between NgModule vs EsModule.
Various metadata options exist for the NgModule metadata decorator.