What is the exact meaning of export keyword in Ang

问题:

I am pretty new in Angular 2. I am studying how to create modules into an Angular app and I have the following doubt related a tutorial that I am following.

My doubt is related to the routing.

So in my example there is defined this AuthModule module:

import { NgModule } from '@angular/core';
import { FormsModule } from '@angular/forms';

import { SigninComponent } from './signin/signin.component';
import { SignupComponent } from './signup/signup.component';
import { AuthRoutingModule } from './auth-routing.module';

@NgModule({
  // Components and directives used by the module:
  declarations: [
    SigninComponent,
    SignupComponent
  ],
  // Import modules used by this features module:
  imports: [
    FormsModule,
    AuthRoutingModule
  ]
})
export class AuthModule {}

and I have the related rotues configuration class defined:

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';

import { ShoppingListComponent } from './shopping-list/shopping-list.component';

const appRoutes: Routes = [
  { path: '', redirectTo: '/recipes', pathMatch: 'full' },
  { path: 'shopping-list', component: ShoppingListComponent }
];

@NgModule({
  imports: [RouterModule.forRoot(appRoutes)],
  exports: [RouterModule]
})
export class AppRoutingModule {

}

So I think that the export keyword means that the content related to this class can be exported and used somewhere else (in this case I think into the imports array of the AuthModule class).

Is it? Or am I missing something? What it the exact meaning of the export statment?

I am not understanding if it is something related to Angular or more generally to TypeScript (because here I found https://www.typescriptlang.org/docs/handbook/modules.html). So it seems to me that this module concept is not directly bounded to Angular 2 framework but is a TypeScript concept to subdivide our code in a smart way (then Angular 2 can use this kind of feature of the language).

Is it or am I missing something?

回答1:

Angular imports/exports and TypeScript imports/exports are two different concepts.

TypeScript imports/exports work at language level to make it clear what a used identifier references exactly. This is entirely unrelated to Angular.

So, if you use FormsModule there can't be any ambiguity, what FormsModule is meant. If there is more than one FormsModule in your code or any of your dependencies, then you need to make it clear with imports which one is meant. You can't import 2 FormsModule from different locations without disambiguation (for example using as foo in the import and then reference it using foo.FormsModule).

This way you can use code from arbitrary 3rd-party libraries and avoid name collisions.

Angular imports/exports are used to make the content of one module available to be used in another module.

Your:

imports: [
    FormsModule,
    AuthRoutingModule
  ]

Allows you to use the directives from FormsModule and AuthRoutingModule in AuthModule and registers the services provided by these modules in the AppModule scope or the closed lazy-loaded root scope.

If you reference any of Angulars directives or services in TypeScript code, you also need to add TypeScript imports. Above FormsModule and AuthRoutingModule need to be imported with TypeScript imports, to make the Angular imports: [...] work.

For example like

<form #f="ngForm">
  <input type="text">
</form>

works only if FormsModule is listed in imports: [ ... ] of your current module.

There is no TypeScript import required, because there is no TypeScript code.

回答2:

Yes you are right by using export keyword before your typescript class you can use that class somewhere else .. in your project