How to setup Braintree API in node js?

I want use braintree for website paymentgateway, unfortunately when I follow the guideline online, & the code unable to be use in node js. Is anything missing?

Index.js:

//send token to clients

app.get("/client_token", function (req, res) {
  gateway.clientToken.generate({}, function (err, response) {
    res.send(response.clientToken);
  });
});

//Receive payment from clients

app.post("/checkout", function (req, res) {
  var nonceFromTheClient = req.body.payment_method_nonce;
  // Use payment method nonce here
});

//Test sandbox
gateway.transaction.sale({
  amount: "10.00",
  paymentMethodNonce: nonceFromTheClient,
  options: {
    submitForSettlement: true
  }
}, function (err, result) {

  if (err) {
    console.error(err);
    return;
  }

  if (result.success) {
    console.log('Transaction ID: ' + result.transaction.id);
  } else {
    console.error(result.message);
  }
});

Error unable pass the node js npm start:

paymentMethodNonce: nonceFromTheClient,

Details in IDE:

ReferenceError: nonceFromTheClient is not defined

at Object.<anonymous> (/Users/desmondkam/codecampvn/index.js:37:23)
at Module._compile (module.js:541:32)
at Object.Module._extensions..js (module.js:550:10)
at Module.load (module.js:456:32)
at tryModuleLoad (module.js:415:12)
at Function.Module._load (module.js:407:3)
at Function.Module.runMain (module.js:575:10)
at startup (node.js:160:18)
at node.js:445:3

via Kimson101