metamask-extension/app/scripts/lib/sig-util.js

const ethUtil = require('ethereumjs-util')

module.exports = {

  concatSig: function (v, r, s) {
    const rSig = ethUtil.fromSigned(r)
    const sSig = ethUtil.fromSigned(s)
    const vSig = ethUtil.bufferToInt(v)
    const rStr = padWithZeroes(ethUtil.toUnsigned(rSig).toString('hex'), 64)
    const sStr = padWithZeroes(ethUtil.toUnsigned(sSig).toString('hex'), 64)
    const vStr = ethUtil.stripHexPrefix(ethUtil.intToHex(vSig))
    return ethUtil.addHexPrefix(rStr.concat(sStr, vStr)).toString('hex')
  },

}

function padWithZeroes (number, length) {
  var myString = '' + number
  while (myString.length < length) {
    myString = '0' + myString
  }
  return myString
}