How to stay Signed In for Gmail api without NodeJs?

I want to get the messages of users by gmail api. For that google authorization is needed. I managed to authorize the user by following code -

      let authBtn = document.getElementById('authorize_button');

      const CLIENT_ID = 'XXXXXX-XXXXXXXXXXX.apps.googleusercontent.com';
      const API_KEY = 'XXXXXX-XXXXXXXXXXXXXXX';

      const DISCOVERY_DOC = 'https://www.googleapis.com/discovery/v1/apis/gmail/v1/rest';

      const SCOPES = 'https://www.googleapis.com/auth/gmail.readonly';

      let tokenClient;
      let gapiInited = false;
      let gisInited = false;

      authBtn.style.visibility = 'hidden';

      function gapiLoaded() {
        gapi.load('client', intializeGapiClient);
      }

      async function intializeGapiClient() {
        await gapi.client.init({
          apiKey: API_KEY,
          discoveryDocs: [DISCOVERY_DOC],
        });
        gapiInited = true;
        maybeEnableButtons();
      }

      function gisLoaded() {
        tokenClient = google.accounts.oauth2.initTokenClient({
          client_id: CLIENT_ID,
          scope: SCOPES,
          callback: '',
        });

        gisInited = true;
        maybeEnableButtons();
      }

      function maybeEnableButtons() {
        if (gapiInited && gisInited) {
          authBtn.style.visibility = 'visible';
        }
      }

      function handleAuthClick() {
        tokenClient.callback = async (resp) => {
          if (resp.error !== undefined) throw (resp);

          authBtn.innerText = 'Refresh';
          await getMessages();
        };

        if (gapi.client.getToken() === null) {
          tokenClient.requestAccessToken({prompt: 'consent'});
        } else {
          tokenClient.requestAccessToken({prompt: ''});
        }
      }

In above code gapi.client.getToken() === null is always false. Everytime I refresh the page I have to reauthorize user with prompt: 'consent'. I also want user to stay signed in until user sign out. How can I achieve by modifying the above code? Can Please someone help me?