tx-decoder

Composable helpers for decoding blockchain transactions

WARNING: This package is deprecated. Use tx-builder which includes both transaction builder and decoder.

This package contains composable helpers for decoding a blockchain transaction and implements an example of bitcoin transaction decoder.

It can be useful for blockchain projects forked from BitcoinCore that want to add custom data to a transaction.

Every composable helper expects a buffer and returns a pair (array of two elements) of a result and a buffer left.

Compared to bitcoinjs-lib which implements decoding in an imperative style where every helper uses buffer of the outer scope and mutates outer offset. Thus its helpers cannot be reused for a different transaction structure.

Example:

Here is how bitcoin transaction decoder is implemented (the main export of the package):

const Buffer = require("safe-buffer").Buffer
const { readInt32, readUInt32 } = require("tx-decoder/src/buffer-utils")
const { compose, addProp } = require("tx-decoder/src/compose")
const { readInputs, readInput, readOutput } = require("tx-decoder/src/tx-decoder")

// Create a buffer from a transaction hex:
const txHex = "0100000001545f6161d2be3bdfe71..."   // see `test/fixture.js` for a full tx example
const buffer = Buffer.from(txHex, 'hex')

// decodeTx :: Buffer -> [Object<version,vin,vout,locktime>, BufferLeft]
const decodeTx = buffer =>
(
  compose([
    addProp('version', readInt32),            // 4 bytes
    addProp('vin', readInputs(readInput)),    // 1-9 bytes (VarInt), Input counter; Variable, Inputs
    addProp('vout', readInputs(readOutput)),  // 1-9 bytes (VarInt), Output counter; Variable, Outputs
    addProp('locktime', readUInt32)           // 4 bytes
  ])({}, buffer)
)

console.log( decodeTx( buffer ) )
// > [{version: 1, vin: [...], vout: [...]}, <Buffer >]

Here is how readInput is defined in the same composable manner:

const { readSlice, readVarSlice, readUInt32 } = require("tx-decoder/src/buffer-utils")
const { compose, addProp } = require("tx-decoder/src/compose")

// readInput :: Buffer -> [Res, Buffer]
const readInput = buffer =>
(
  compose([
    addProp('hash', readSlice(32)),           // 32 bytes, Transaction Hash
    addProp('index', readUInt32),             // 4 bytes, Output Index
    addProp('script', readVarSlice),          // 1-9 bytes (VarInt), Unlocking-Script Size; Variable, Unlocking-Script
    addProp('sequence', readUInt32)           // 4 bytes, Sequence Number
  ])({}, buffer)
)

Checkout tests/manual-decode.js if you are not familiar with how to decode a transaction. It has a step-by-step imperative-style example.

Checkout tests for exact examples of how to use the package helpers.

Upcoming

• decode script
• create a transaction builder (see tx-builder package)

Release Notes:

• 0.2.0 Added readHash helper
  • Reverse the hash (tx id) after its read from hex.
• 0.1.2 Initial version
  • buffer utils, compose helpers, bitcoin tx decode example.