An Angular interceptor to parse string dates (ISO8601 format) from server response to JS Date Object. There are both RxJs 5 and RxJs 6 versions

AngularRxJs5DateHttpInterceptor.ts

import { Injectable } from '@angular/core';
import { HttpRequest, HttpHandler, HttpEvent, HttpInterceptor, HttpErrorResponse, HttpResponse } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/do';

@Injectable()
export class AngularDateHttpInterceptor implements HttpInterceptor {
  // Migrated from AngularJS https://raw.githubusercontent.com/Ins87/angular-date-interceptor/master/src/angular-date-interceptor.js
  iso8601 = /^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d(\.\d+)?(([+-]\d\d:\d\d)|Z)?$/;

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    return next.handle(req).do((event: HttpEvent<any>) => {
      if (event instanceof HttpResponse) {
        const body = event.body;
        this.convertToDate(body);
      }
    }, (err: any) => {
      if (err instanceof HttpErrorResponse) {
        if (err.status === 401) {
        }
      }
    });
  }

  convertToDate(body) {
    if (body === null || body === undefined) {
      return body;
    }

    if (typeof body !== 'object') {
      return body;
    }

    for (const key of Object.keys(body)) {
      const value = body[key];
      if (this.isIso8601(value)) {
        body[key] = new Date(value);
      } else if (typeof value === 'object') {
        this.convertToDate(value);
      }
    }
  }

  isIso8601(value) {
    if (value === null || value === undefined) {
      return false;
    }

    return this.iso8601.test(value);
  }
}

AngularRxJs6DateHttpInterceptor.ts

import {
    HttpRequest,
    HttpHandler,
    HttpEvent,
    HttpInterceptor,
    HttpErrorResponse,
    HttpResponse
} from '@angular/common/http';
import { Observable } from 'rxjs';
import { tap } from 'rxjs/operators';

export class AngularDateHttpInterceptor implements HttpInterceptor {
    // Migrated from AngularJS https://raw.githubusercontent.com/Ins87/angular-date-interceptor/master/src/angular-date-interceptor.js
    iso8601 = /^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d(\.\d+)?(([+-]\d\d:\d\d)|Z)?$/;

    public intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        return next.handle(req).pipe(
			tap({
				next: (event: HttpEvent<any>) => {
                if (event instanceof HttpResponse) {
                    const body = event.body;
                    this.convertToDate(body);
                }
				},
				error: (err: any) => {
                if (err instanceof HttpErrorResponse) {
                    if (err.status === 401) {
                    }
                }
				},
            }),
        );
    }

    convertToDate(body) {
		if (typeof body === 'object' && body !== null) {
			const casted = body as { [key: string]: unknown };
			for (const key of Object.keys(casted)) {
				const value = casted[key];
            if (this.isIso8601(value)) {
					casted[key] = new Date(value);
				} else if (typeof value === 'object' && value !== null) {
                this.convertToDate(value);
            }
        }
		} else {
			return body;
    	}
    }

	isIso8601(value: unknown): value is string {
		return typeof value === 'string' ? this.iso8601.test(value) : false;
    }
}