Get attachement file by google api in nodejs

To retrieve an attachment file using the Google API in Node.js, you can use the gmail.users().messages().attachments.get method. This method requires the ID of the message and the ID of the attachment, which you can obtain from the payload field of the message resource.

Here is an example of how to use this method to retrieve an attachment and save it to a local file:

const fs = require('fs');
const {google} = require('googleapis');

async function getAttachment(auth, messageId, attachmentId) {
  const gmail = google.gmail({version: 'v1', auth});
  const attachment = await gmail.users().messages().attachments().get({
    userId: 'me',
    messageId: messageId,
    id: attachmentId
  });
  const buffer = Buffer.from(attachment.data, 'base64');
  fs.writeFileSync('attachment.pdf', buffer);
}

Note that this example assumes that you have already authenticated your application and obtained an authorized client object (auth). You will also need to specify the user ID (in this case, 'me') and the IDs of the message and attachment.

You can find more information about working with attachments in the Gmail API documentation:

https://developers.google.com/gmail/api/reference/rest/v1/users.messages.attachments/get