How to obtain access token for Graph API

get token for Graph API

import * as qs from 'querystring';
import 'isomorphic-fetch';
import {
    AuthenticationHandler,
    AuthenticationProviderOptions,
    Client,
    HTTPMessageHandler
} from '@microsoft/microsoft-graph-client';

export function getClient() {
    const authProvider = {
        getAccessToken: (authenticationProviderOptions?: AuthenticationProviderOptions): Promise<string> => {
                        const params = {
                            client_id: process.env.CLIENT_ID,
                            scope: 'https://graph.microsoft.com/.default',
                            client_secret: process.env.CLIENT_SECRET,
                            grant_type: 'client_credentials'
                        };

                        const body = qs.stringify(params);
                        return fetch(`https://login.microsoftonline.com/${process.env.TENANT_ID}/oauth2/v2.0/token`, {
                            method: 'POST',
                            headers: {
                                'Content-Type': 'application/x-www-form-urlencoded'
                            },
                            body
                        }).then(response => {
                            if (response.status >= 400) {
                                throw new Error(`Bad response from server: ${response.statusText}`);
                            }
                            return response.json();
                        }).then(body => {
                            return (body as unknown as {access_token: string}).access_token;
                        });
        }
    };
    const authHandler = new AuthenticationHandler(authProvider);
    const httpMessageHandler = new HTTPMessageHandler();

    authHandler.setNext(httpMessageHandler);
    return Client.initWithMiddleware({
        defaultVersion: 'v1.0',
        debugLogging: true,
        middleware: authHandler,
    });
}