Below show the method to Integrate the Braintree Payment gateway in MEAN Stack while developing software.

The Braintree Node.js library with Braintree’s servers.
Prerequisite:
Install Braintree node package:
For more information or help with this step, see our API Credentials support article.

In Client Controller:

braintree.client.create({
	authorization:'*******'//provide authorization code
}, 
function (clientErr, clientInstance) {
//Stop if there was a problem creating the client.
		//This could happen if there is a network error or if the authorization is invalid.
		if (clientErr) {
			console.error('Error creating client:', clientErr);
			return;}
//Create a PayPal Checkout component.
  		braintree.paypalCheckout.create({
    		client: clientInstance
  		},function (paypalCheckoutErr,paypalCheckoutInstance){
//Stop if there was a problem creating PayPal Checkout.
		//This could happen if there was a network error or if it's incorrectly configured.
		if(paypalCheckoutErr){
			console.error('Error creating PayPal Checkout:',   paypalCheckoutErr);
 			return;}
//Set up PayPal with the checkout.js library
		paypal.Button.render({
		env: 'sandbox', // or 'production'
		commit: true,
payment: function (){
return paypalCheckoutInstance.createPayment({
flow:'checkout',//Required
amount: $scope.totalAmt,//Required,Provide amount here
					currency:'USD',//Required
					enableShippingAddress:true,
					shippingAddressEditable:false,
				});
			},
onAuthorize:function (data, actions){
      return paypalCheckoutInstance.tokenizePayment(data)
	  .then(function (payload){
	   // Submit`payload.nonce`to your server.
					$http.post('/api/braintree', 
{params: {braintreeData: data, payload: payload,amount: $scope.totalAmt }})//post to backend nodejs
    				.success(
        			function(success){
console.log(success); //Perform success operation here})
    				.error(
        			function(error){
					console.log(error);});
				});},
onCancel: function (data){
			console.log(data);
			},onError: function (err){
			console.error('checkout.js error',err);
			}},'#paypal-button').then(function(){
				// The PayPal button will be rendered in an html element with the id
				//`paypal-button`.This function will be called when the PayPal button
				//is set up and ready to be used.
			});
  	});
});

In the backend server controller

var braintree = require('braintree');
app.post("/api/braintree", function (req, res) {
	
  	var payload = req.body.params.payload;
  	var braintreeData = req.body.params.braintreeData;
  	var totAmount =  req.body.params.amount;
  	
	var saleRequest = {
		amount: totAmount,
		paymentMethodNonce: payload.nonce,
		orderId: braintreeData.orderID,
		options: {
			submitForSettlement: true,
			paypal: {
				customField: "PayPal custom field",
      		description: "Description for PayPal email receipt",
    		},}
	};
var gateway = braintree.connect({
  		environment: braintree.Environment.Sandbox, // Sandbox Environment
  		merchantId: "*********", 	// Provide merchant id
  		publicKey: "*********", 	// Public key
  		privateKey: "*********" 	// Private key
	});
  
	gateway.transaction.sale(saleRequest, function (err, result) {
  		if (err) {
    		res.send("Error:  " + err + "");
  		} else if (result.success) {
  			// Perform any database operation here
    		res.json(result);
  		} else {
    		res.send("Error:  " + result.message + "");
  		}
	});
});