btc-base-api.js

{
  async createTransaction(address = '', amount = 0, fee) {
    const unspents = await this.getUnspents()

    // populate with txHex <--
    for (const unspent of unspents) {
      const txHex = await this._get(`/tx/${unspent.txid}/hex`)
      unspent.txHex = txHex
    }

    const hex = this._buildTransaction(address, amount, unspents, fee)

    let txid = bitcoin.crypto.sha256(Buffer.from(hex, 'hex'))
    txid = bitcoin.crypto.sha256(Buffer.from(txid))
    txid = txid.toString('hex').match(/.{2}/g).reverse().join('')

    return { hex, txid }
  },
    
  _buildTransaction(address, amount, unspents, fee) {
    amount = new BigNumber(amount).times(this.multiplier).toNumber()
    amount = Math.floor(amount)

    const txb = new bitcoin.Psbt({
      network: this._network
    })
    txb.setVersion(1)

    const target = amount + new BigNumber(fee).times(this.multiplier).toNumber()
    let transferAmount = 0
    let inputs = 0

    unspents.forEach((tx) => {
      const amt = Math.floor(tx.amount)
      if (transferAmount < target) {
        txb.addInput({
          hash: tx.txid,
          index: tx.vout,
          nonWitnessUtxo: Buffer.from(tx.txHex, 'hex')
        })
        transferAmount += amt
        inputs++
      }
    })

    txb.addOutput({
      address,
      value: amount
    })
    // This is a necessary step
    // If we'll not add a change to output, it will burn in hell
    const change = transferAmount - target
    if (isPositiveNumber(change)) {
      txb.addOutput({
        address: this._address,
        value: change
      })
    }

    for (let i = 0; i < inputs; ++i) {
      txb.signInput(i, this._keyPair)
    }

    txb.finalizeAllInputs()
    const tx = txb.extractTransaction()

    return tx.toHex()
  }
}