index.ts

// Example in Typescript, nodejs reads a Google Doc if the service account is added as a viewer
// No manual OAUTH2 approval steps required. Just use the google share settings to share
// Use the client_email address found in the crendentials file as the 
import fs = require('fs');
import {JWT, auth} from 'google-auth-library';
import {google} from 'googleapis';

const SCOPES = [
     // Other options at https://developers.google.com/identity/protocols/oauth2/scopes#docsv1
    'https://www.googleapis.com/auth/documents.readonly'
];

// Service account key, see https://cloud.google.com/iam/docs/creating-managing-service-account-keys#creating_service_account_keys
// If you deploy to GCF, Cloud Run you may be able to pick up credentials from the environment and not need a file
fs.readFile('creds/assisty.json', (err, crendentials) => {
    if (err) return console.log('Error loading service credentials:', err);
    const client = <JWT> auth.fromJSON(JSON.parse(crendentials.toString()));
    client.scopes = SCOPES;
    run(client);
});


async function run(auth: JWT) {
    // https://developers.google.com/docs/api/reference/rest
    const docs = google.docs({
      version: 'v1',
      auth
    });
    const doc = await docs.documents.get({
      documentId: '18uEHALOrqvh9Nf4yosKPZDdeKZV6nfk8Ddd4cEXjgfA',
    });
    console.log(`${JSON.stringify(doc.data.body)}`);
  }

package.json

{
  "name": "Google Doc reading example",
  "version": "1.0.0",
  "description": "A simple Node.js command-line application that reads a doc through the Docs API.",
  "dependencies": {
    "google-auth-library": "^5.10.1",
    "googleapis": "^48.0.0"
  },
  "devDependencies": {
    "@types/node": "^13.9.2",
    "typescript": "^3.8.3"
  }
}

No need to read the service account key json file manually. You can specify its path in the GOOGLE_APPLICATION_CREDENTIALS environment variable, or keyFile property, as explained here: https://googleapis.dev/nodejs/googleapis/latest/sheets/index.html#service-account-credentials