Retrieving Access Token using Service Account for Node.js without using googleapis

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This is a sample Node.js script to retrieve access token from Service Account of Google without using googleapis.

const cryptor = require('crypto');
const request = require('request');

const privateKey = "###"; // private_key of JSON file retrieved by creating Service Account
const clientEmail = "###"; // client_email of JSON file retrieved by creating Service Account
const scopes = ["https://www.googleapis.com/auth/drive.readonly"]; // Sample scope

const url = "https://www.googleapis.com/oauth2/v4/token";
const header = {
  alg: "RS256",
  typ: "JWT",
};
const now = Math.floor(Date.now() / 1000);
const claim = {
  iss: clientEmail,
  scope: scopes.join(" "),
  aud: url,
  exp: (now + 3600).toString(),
  iat: now.toString(),
};

const signature = Buffer.from(JSON.stringify(header)).toString('base64') + "." + Buffer.from(JSON.stringify(claim)).toString('base64');

var sign = cryptor.createSign('RSA-SHA256');
sign.update(signature);
const jwt = signature + "." + sign.sign(privateKey, 'base64');

request({
  method: "post",
  url: url,
  body: JSON.stringify({
    assertion: jwt,
    grant_type: "urn:ietf:params:oauth:grant-type:jwt-bearer",
  }),
}, (err, res, body) => {
  if (err) {
    console.log(err);
    return;
  }
  console.log(body);
});

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