An interface cannot be exported in an angular 2 module?

Question

I tried to export an interface in a NgModule-declaration and export and getting this error already in the editor (Visual Studio Code): [ts] 'MyInterface' only refers to a type, but is being used as a value here.

Here is the example code Edit-1:

import { NgModule }           from '@angular/core'; import { CommonModule }       from '@angular/common'; import { FormsModule }        from '@angular/forms'; import { MaterialModule }     from '@angular/material'; import { MyInterface }        from './my.interface'; import { MyService }          from './my.service';  @NgModule({   imports:      [ CommonModule, FormsModule,  MaterialModule.forRoot()  ],   declarations: [ MyInterface],//<- this is causing the message   exports:      [ MyInterface],   providers:    [ MyService ] }) export class MyModule { } 

One part of an explanation I found in the answer to this post: "since interfaces are erased at runtime in TypeScript". I'm currently refactoring my app to feature modules, so I cannot test it right now: Can I use the interfaces only by import from './mypathto/my.interface'?

Answer 1

You cannot export an interface. You can only export:

1. Other modules
2. Components
3. Directives
4. Pipes

NgModule is an angular concept and should not be confused with a typescript module. To make a third party, who uses your module, able to use your interface, you should create a .d.ts definition file with your module.

If you want to use a interface inside another NgModule of yours, you should just use:

import {InterfaceName} from 'path/to/ngmodule/to/interface'; 

Also, do not put an interface in the declarations array, this is only used for pipes/components/directives.

If you want your interface to be usable outside of an library, you should add it to the export of your index.ts:

export {InterfaceName} from 'path/to/ngmodule/to/interface';