How to export const with NgModule

Question

I have one constant which should be usable by other modules importing mine.

I have Datepicker directive which has defined date formats constants (I have it like this so someone using my directive can use these constants):

export const DATETIME_FORMAT = 'DD/MM/YYYY HH:mm:ss';
export const DATE_FORMAT = 'DD/MM/YYYY';
export const TIME_FORMAT = 'DD/MM/YYYY';

And NgModule for exporting them further. This module is built and then used in other projects/modules.

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { DatepickerDirective, TIME_FORMAT, DATETIME_FORMAT, DATE_FORMAT } from './datepicker.directive';

@NgModule({
  imports: [
    CommonModule
  ],
  exports: [
    DatepickerDirective,
    DATE_FORMAT,
    DATETIME_FORMAT,
    TIME_FORMAT
  ],
  declarations: [
    DatepickerDirective
  ]
})
export class DatepickerModule { }

But when trying to build it I get error:

ERROR in src/app/datepicker/datepicker.module.ts(5,11): error TS2345: Argument of type '{ imports: (typeof CommonModule)[]; exports: (string | typeof DatepickerDirective)[]; declara...' is not assignable to parameter of type 'NgModule'.
  Types of property 'exports' are incompatible.
    Type '(string | typeof DatepickerDirective)[]' is not assignable to type '(any[] | Type<any>)[]'.
      Type 'string | typeof DatepickerDirective' is not assignable to type 'any[] | Type<any>'.
        Type 'string' is not assignable to type 'any[] | Type<any>'.

Answer 1

NgModule can't exports constants but you can provides the values with InjectionToken. Find more about InjectionToken here.

import { ModuleWithProviders, NgModule, InjectionToken } from '@angular/core';
import { CommonModule } from '@angular/common';
import { DatepickerDirective, TIME_FORMAT, DATETIME_FORMAT, DATE_FORMAT } from './datepicker.directive';

export interface IDatepickerFormats {
  TIME_FORMAT: string;
  DATETIME_FORMAT: string;
  DATE_FORMAT: string;
}

export const DATEPICKER_FORMATS = new InjectionToken<IDatepickerFormats>('datepicker.formats');

@NgModule({
  imports: [CommonModule],
  exports: [DatepickerDirective],
  declarations: [DatepickerDirective]
})
export class DatepickerModule {
  static forRoot(): ModuleWithProviders {
    return {
      ngModule: DatepickerModule,
      providers: [
        {
          provide: DATEPICKER_FORMATS,
          useValue: {TIME_FORMAT, DATETIME_FORMAT, DATE_FORMAT}
        }
      ]
    };
  }
}

Now, every module whose importing Datepicker module can inject and using these constants in his declared components. For example:

import { Component, Inject} from '@angular/core';
import { DATEPICKER_FORMATS, IDatepickerFormats } from './datepicker';

@Component({
  selector: 'app-example',
  templateUrl: './app-example.component.html',
  styleUrls: ['./app-example.component.scss']
})
export class AppExampleComponent {
  private DATE_FORMAT: string;
  private TIME_FORMAT: string;
  private DATETIME_FORMAT: string;

  constructor(@Inject(DATEPICKER_FORMATS) private datePickerFormats: IDatepickerFormats) {
    this.DATE_FORMAT = this.datePickerFormats.DATE_FORMAT;
    this.TIME_FORMAT = this.datePickerFormats.TIME_FORMAT;
    this.DATETIME_FORMAT = this.datePickerFormats.DATETIME_FORMAT;
  }
}