What is the difference between declarations, providers, and import in NgModule?

I am trying to understand Angular (sometimes called Angular2+), then I came across @Module:

1. Imports

2. Declarations

3. Providers

Following Angular Quick Start

Angular Concepts

• imports makes the exported declarations of other modules available in the current module
• declarations are to make directives (including components and pipes) from the current module available to other directives in the current module. Selectors of directives, components or pipes are only matched against the HTML if they are declared or imported.
• providers are to make services and values known to DI (dependency injection). They are added to the root scope and they are injected to other services or directives that have them as dependency.

A special case for providers are lazy loaded modules that get their own child injector. providers of a lazy loaded module are only provided to this lazy loaded module by default (not the whole application as it is with other modules).

For more details about modules see also https://angular.io/docs/ts/latest/guide/ngmodule.html

• exports makes the components, directives, and pipes available in modules that add this module to imports. exports can also be used to re-export modules such as CommonModule and FormsModule, which is often done in shared modules.

• entryComponents registers components for offline compilation so that they can be used with ViewContainerRef.createComponent(). Components used in router configurations are added implicitly.

TypeScript (ES2015) imports

import ... from 'foo/bar' (which may resolve to an index.ts) are for TypeScript imports. You need these whenever you use an identifier in a typescript file that is declared in another typescript file.

Angular's @NgModule() imports and TypeScript import are entirely different concepts.

See also jDriven - TypeScript and ES6 import syntax

> Most of them are actually plain ECMAScript 2015 (ES6) module syntax that TypeScript uses as well.

imports are used to import supporting modules like FormsModule, RouterModule, CommonModule, or any other custom-made feature module.

declarations are used to declare components, directives, pipes that belong to the current module. Everyone inside declarations knows each other. For example, if we have a component, say UsernameComponent, which displays a list of the usernames and we also have a pipe, say toupperPipe, which transforms a string to an uppercase letter string. Now If we want to show usernames in uppercase letters in our UsernameComponent then we can use the toupperPipe which we had created before but the question is how UsernameComponent knows that the toupperPipe exists and how it can access and use that. Here come the declarations, we can declare UsernameComponent and toupperPipe.

Providers are used for injecting the services required by components, directives, pipes in the module.

Components are declared, Modules are imported, and Services are provided. An example I'm working with:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';


import { AppComponent } from './app.component';
import {FormsModule} from '@angular/forms';
import { UserComponent } from './components/user/user.component';
import { StateService } from './services/state.service';    

@NgModule({
  declarations: [
    AppComponent,
    UserComponent
  ],
  imports: [
    BrowserModule,
    FormsModule
  ],
  providers: [ StateService ],
  bootstrap: [ AppComponent ]
})
export class AppModule { }
 

Adding a quick cheat sheet that may help after the long break with Angular:

---

DECLARATIONS

Example:

declarations: [AppComponent]

What can we inject here? Components, pipes, directives

---

IMPORTS

Example:

imports: [BrowserModule, AppRoutingModule]

What can we inject here? other modules

---

PROVIDERS

Example:

providers: [UserService]

What can we inject here? services

---

BOOTSTRAP

Example:

bootstrap: [AppComponent]

What can we inject here? the main component that will be generated by this module (top parent node for a component tree)

---

ENTRY COMPONENTS

Example:

entryComponents: [PopupComponent]

What can we inject here? dynamically generated components (for instance by using ViewContainerRef.createComponent())

---

EXPORT

Example:

export: [TextDirective, PopupComponent, BrowserModule]

What can we inject here? components, directives, modules or pipes that we would like to have access to them in another module (after importing this module)

Angular @NgModule constructs:

1. import { x } from 'y';: This is standard typescript syntax (ES2015/ES6 module syntax) for importing code from other files. This is not Angular specific. Also this is technically not part of the module, it is just necessary to get the needed code within scope of this file.
2. imports: [FormsModule]: You import other modules in here. For example we import FormsModule in the example below. Now we can use the functionality which the FormsModule has to offer throughout this module.
3. declarations: [OnlineHeaderComponent, ReCaptcha2Directive]: You put your components, directives, and pipes here. Once declared here you now can use them throughout the whole module. For example we can now use the OnlineHeaderComponent in the AppComponent view (html file). Angular knows where to find this OnlineHeaderComponent because it is declared in the @NgModule.
4. providers: [RegisterService]: Here our services of this specific module are defined. You can use the services in your components by injecting with dependency injection.

Example module:

// Angular
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';

// Components
import { AppComponent } from './app.component';
import { OfflineHeaderComponent } from './offline/offline-header/offline-header.component';
import { OnlineHeaderComponent } from './online/online-header/online-header.component';

// Services
import { RegisterService } from './services/register.service';

// Directives
import { ReCaptcha2Directive } from './directives/re-captcha2.directive';

@NgModule({
  declarations: [
    OfflineHeaderComponent,,
    OnlineHeaderComponent,
    ReCaptcha2Directive,
    AppComponent
  ],
  imports: [
    BrowserModule,
    FormsModule,
  ],
  providers: [
    RegisterService,
  ],
  entryComponents: [
    ChangePasswordComponent,
    TestamentComponent,
    FriendsListComponent,
    TravelConfirmComponent
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

1. declarations: This property tells about the Components, Directives and Pipes that belong to this module.
2. exports: The subset of declarations that should be visible and usable in the component templates of other NgModules.
3. imports: Other modules whose exported classes are needed by component templates declared in this NgModule.
4. providers: Creators of services that this NgModule contributes to the global collection of services; they become accessible in all parts of the app. (You can also specify providers at the component level, which is often preferred.)
5. bootstrap: The main application view, called the root component, which hosts all other app views. Only the root NgModule should set the bootstrap property.