@ngx-translate/core/lib/missing-translation-handler.ts

import {Injectable} from "@angular/core";
import {TranslateService} from "./translate.service";

export interface MissingTranslationHandlerParams {
  /**
   * the key that's missing in translation files
   */
  key: string;

  /**
   * an instance of the service that was unable to translate the key.
   */
  translateService: TranslateService;

  /**
   * interpolation params that were passed along for translating the given key.
   */
  interpolateParams?: Object;
}

export abstract class MissingTranslationHandler {
  /**
   * A function that handles missing translations.
   *
   * @param params context for resolving a missing translation
   * @returns a value or an observable
   * If it returns a value, then this value is used.
   * If it return an observable, the value returned by this observable will be used (except if the method was "instant").
   * If it doesn't return then the key will be used as a value
   */
  abstract handle(params: MissingTranslationHandlerParams): any;
}

/**
 * This handler is just a placeholder that does nothing, in case you don't need a missing translation handler at all
 */
@Injectable()
export class FakeMissingTranslationHandler implements MissingTranslationHandler {
  handle(params: MissingTranslationHandlerParams): string {
    return params.key;
  }
}