How to get refresh token while using Google API JS Client

It does not return a refresh token as designed. The tutorial and the code you mentioned are using Google APIs Client Library for JavaScript. This library uses the OAuth 2.0 client-side flow for making requests that require authorization.

As The OAuth 2.0 Authorization Framework says:

The implicit grant type is used to obtain access tokens (it does not support the issuance of refresh tokens) and is optimized for public clients known to operate a particular redirection URI. These clients are typically implemented in a browser using a scripting language such as JavaScript.

In fact, The authorization code flow is the only one which issue refresh token, and Google supports this flow in these Scenarios: Web server applications, Installed applications, and Applications on limited-input devices, but not Client-side (JavaScript) applications or Service accounts. Get more details from here.

So you'll not get refresh token in this way.

Not sure if this is the right way, but I'm able to get a refresh token using the following code:

window.gapi.client.init({
  apiKey: this.GOOGLE.API_KEY,
  clientId: this.GOOGLE.CLIENT_ID,
  discoveryDocs: DISCOVERY_DOCS,
  scope: SCOPES
}).then(()=>{
  const authInstance = window.gapi.auth2.getAuthInstance();
  authInstance.grantOfflineAccess()
   .then((res) => {
      console.log(res);
      this.data.refreshToken = res.code;
   });
});