Get Google JWT token.

gistfile1.js

var fs = require('fs');
var jwt = require('jsonwebtoken');
var request = require('request');

// constants
var GOOGLE_OAUTH2_URL = 'https://accounts.google.com/o/oauth2/token';

var iat = Math.floor(Date.now() / 1000);
var exp = iat + 60 * 60;
var payload = {
  iss: '123520686343-k6tfn73sentmh0ss5nu67kniorbcta8n@developer.gserviceaccount.com',
  scope: 'https://www.googleapis.com/auth/devstorage.full_control',
  aud: GOOGLE_OAUTH2_URL,
  exp: exp,
  iat: iat
};
console.log('request payload', payload);
// sign with RSA SHA256
var cert = fs.readFileSync('google_cloud_key.pem'); // get private key
var token = jwt.sign(payload, cert, { algorithm: 'RS256' });
request.post(GOOGLE_OAUTH2_URL, {
  headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
  form: {
    grant_type: 'urn:ietf:params:oauth:grant-type:jwt-bearer',
    assertion: token
  }
}, function(err, res, body) {
  if (err) {
    console.log(err);
  } else {
    console.log('response from OAuth server');
    console.log('http status', res.statusCode);
    console.log('body', body);
  }
  if (res.statusCode != 200) {
    err = new Error(
      'failed to obtain an authentication token, request failed with HTTP code ' +
      res.statusCode + ': ' + body.error
    );
    err.statusCode = res.statusCode;
    err.body = body;
    console.log(err);
  } else {
    try {
      body = JSON.parse(body);
    } catch (e) {
      console.log(new Error('failed to parse response body: ' + body));
    }
    console.log(body);
  }
});