Script to spend an UTXO to a single address, with custom fee rate

self-transfer.js

#! /usr/bin/env node

// Instructions: place the file inside a directory, run `npm init -y` and install dependencies with
// `npm i bitcoinjs-lib@^6.1.1 ecpair@^2.1.0 tiny-secp256k1@^2.2.1`

const bitcoin = require("bitcoinjs-lib");
const ecc = require("tiny-secp256k1");
const { ECPairFactory } = require("ecpair");
const ECPair = ECPairFactory(ecc);
bitcoin.initEccLib(ecc);

// fill with correct/desired values
const network = bitcoin.networks.bitcoin;
const wif = "<privkey>";
const inputValue = 3000;
const feeRate = 0.1;
const sendToAddr = "<destination_addr>";
const txid = "<txid>";
const index = 0;

const keyPair = ECPair.fromWIF(wif, network);
const internalPubkey = keyPair.publicKey.subarray(1);
const { output } = bitcoin.payments.p2tr({ internalPubkey, network });

const tweakedKeyPair = keyPair.tweak(
    bitcoin.crypto.taggedHash('TapTweak', internalPubkey),
);

const dummyPsbt = new bitcoin.Psbt({ network });
dummyPsbt.addInput({
    hash: txid,
    index,
    witnessUtxo: {
        script: output,
        value: inputValue
    },
    tapInternalKey: internalPubkey
});

dummyPsbt.addOutput({
    address: sendToAddr,
    value: inputValue
});

dummyPsbt.signAllInputs(tweakedKeyPair);
dummyPsbt.finalizeAllInputs();

const size = dummyPsbt.extractTransaction().virtualSize();
const fee = Math.ceil(size * feeRate);

const finalPsbt = new bitcoin.Psbt({ network });
finalPsbt.addInput({
    hash: txid,
    index,
    witnessUtxo: {
        script: output,
        value: inputValue
    },
    tapInternalKey: internalPubkey
});

finalPsbt.addOutput({
    address: sendToAddr,
    value: inputValue - fee
});

finalPsbt.signAllInputs(tweakedKeyPair);
finalPsbt.finalizeAllInputs();

const finalTx = finalPsbt.extractTransaction();
const actualSize = finalTx.virtualSize();
const actualFeeRate = (fee / actualSize).toFixed(3);

console.log(`Size: ${actualSize} vb`);
console.log(`Fee: ${fee} sats`);
console.log(`Fee rate: ${actualFeeRate} sat/vb`);

console.log("\nTransaction hex:")
console.log(finalTx.toHex());