Integrating the Braintree payment gateway in a MEAN (MongoDB, Express, Angular, Node.js) stack involves several steps. Below is 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 + "");
  		}
	});
});

Run the Server

Make sure your Node.js server is running:

node server.js

Conclusion

You have now integrated the Braintree payment gateway into your MEAN stack application. This setup will allow you to handle payments using PayPal via Braintree. Make sure to handle any sensitive information securely and follow best practices for handling payment gateways in your production environment.