Postman auto fetch oAuth access token

pre-request-scripts.js

// Add this script to collection's pre-request scripts
// Collection -> Edit -> Pre-request Scripts

// Automatically fetch oAuth access_token and store it for 1 hour
update_token = pm.environment.get("token_time");
if (typeof(update_token) == "undefined" || update_token <= Date.now()) {
  console.log("Fetching new oAuth access token for user");

  pm.sendRequest({
    url: pm.environment.get("host") + pm.environment.get("token_path"),
    method: 'POST',
    header: {},
    body: {
      mode: 'urlencoded',
      urlencoded: [
        { key: "grant_type", value: "password" },
        { key: "client_id", value: pm.environment.get("client_id") },
        { key: "client_secret", value: pm.environment.get("client_secret") },
        { key: "username", value: pm.environment.get("username") },
        { key: "password", value: pm.environment.get("password") },
      ]
    }
  }, function (err, res) {
    if (res.code == 200) {
      var expireIn = 60 * 60 * 1000; // 1 hour in milliseconds
      pm.environment.set("admin_access_token", res.json().access_token);
      pm.environment.set("token_time", Date.now() + expireIn);
    }
    else {
      console.log(res.json());
      pm.environment.set("token_time", null);
      pm.environment.set("admin_access_token", null);
    }
  });
}
else {
  console.log("Reusing existing oAuth access token");
}