Angular Intercept Check refresh token

 https://medium.com/angular-in-depth/top-10-ways-to-use-interceptors-in-angular-db450f8a62d6

import { Injectable } from "@angular/core";

import {
	HttpEvent, HttpInterceptor, HttpHandler,
	HttpRequest, HttpErrorResponse
	} from "@angular/common/http";
	import { throwError, Observable, BehaviorSubject, of } from "rxjs";
	import { catchError, filter, take, switchMap } from "rxjs/operators";
	@Injectable()
	export class AuthInterceptor implements HttpInterceptor {
	private AUTH_HEADER = "Authorization";
	private token = "secrettoken";
	private refreshTokenInProgress = false;
	private refreshTokenSubject: BehaviorSubject<any> = new BehaviorSubject<any>(null);
	intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
	if (!req.headers.has('Content-Type')) {
	req = req.clone({
	headers: req.headers.set('Content-Type', 'application/json')
	});
	}
	req = this.addAuthenticationToken(req);
	return next.handle(req).pipe(
	catchError((error: HttpErrorResponse) => {
	if (error && error.status === 401) {
	// 401 errors are most likely going to be because we have an expired token that we need to refresh.
	if (this.refreshTokenInProgress) {
	// If refreshTokenInProgress is true, we will wait until refreshTokenSubject has a non-null value
	// which means the new token is ready and we can retry the request again
	return this.refreshTokenSubject.pipe(
	filter(result => result !== null),
	take(1),
	switchMap(() => next.handle(this.addAuthenticationToken(req)))
	);
	} else {
	this.refreshTokenInProgress = true;
	// Set the refreshTokenSubject to null so that subsequent API calls will wait until the new token has been retrieved
	this.refreshTokenSubject.next(null);
	return this.refreshAccessToken().pipe(
	switchMap((success: boolean) => {
	this.refreshTokenSubject.next(success);
	return next.handle(this.addAuthenticationToken(req));
	}),
	// When the call to refreshToken completes we reset the refreshTokenInProgress to false
	// for the next time the token needs to be refreshed
	finalize(() => this.refreshTokenInProgress = false)
	);
	}
	} else {
	return throwError(error);
	}
	})
	);
	}
	private refreshAccessToken(): Observable<any> {
	return of("secret token");
	}
	private addAuthenticationToken(request: HttpRequest<any>): HttpRequest<any> {
	// If we do not have a token yet then we should not set the header.
	// Here we could first retrieve the token from where we store it.
	if (!this.token) {
	return request;
	}
	// If you are calling an outside domain then do not add the token.
	if (!request.url.match(/www.mydomain.com\//)) {
	return request;
	}
	return request.clone({
	headers: request.headers.set(this.AUTH_HEADER, "Bearer " + this.token)
	});
	}
	}