Understanding signing messages with bitcoinjs-lib

I’m trying to understand what a specific line of code does when signing a message using bitcoinjs-lib. Here’s the code:

var bitcoin = require('bitcoinjs-lib') var bitcoinMessage = require('bitcoinjs-message')  var keyPair = bitcoin.ECPair.makeRandom(); //private key var privateKey = keyPair.toWIF(); // Address var address = keyPair.getAddress();  console.log(privateKey); console.log(address);  privateKey = keyPair.d.toBuffer(32) var message = 'your message' var messagePrefix = bitcoin.networks.bitcoin.messagePrefix  var signature = bitcoinMessage.sign(message, messagePrefix, privateKey, keyPair.compressed) console.log(signature.toString('base64')) 

The line I’m curious about is this one:

privateKey = keyPair.d.toBuffer(32) 

What exactly is this code doing and why is it necessary?

Thanks,

Connor

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