read-git-index.js

const fs = require('fs')
const index = fs.readFileSync('.git/index')
const result = {}

let p = 0
result.signature = index.slice(p, p + 4).toString('ascii')
p += 4
if (result.signature !== 'DIRC') {
  throw new Error('not a git index file')
}

result.version = index.readUInt32BE(p)
p += 4
if (result.version !== 3 && result.version !== 2) {
  new Error('unsupported git index version: ' + result.version)
}
result.entries = new Array(index.readUInt32BE(p))
p += 4
console.error('%j entries', result.entries.length)

for (let i = 0; i < result.entries.length && p < index.length; i ++) {
  const entry = result.entries[i] = { offset: p }
  entry.entry = i + 1
  entry.ctimeSeconds = index.readUInt32BE(p)
  p += 4
  entry.ctimeNanoseconds = index.readUInt32BE(p)
  p += 4
  entry.mtimeSeconds = index.readUInt32BE(p)
  p += 4
  entry.mtimeNanoseconds = index.readUInt32BE(p)
  p += 4
  entry.dev = index.readUInt32BE(p)
  p += 4
  entry.ino = index.readUInt32BE(p)
  p += 4
  entry.mode = index.readUInt32BE(p).toString(8)
  p += 4
  entry.uid = index.readUInt32BE(p)
  p += 4
  entry.gid = index.readUInt32BE(p)
  p += 4
  entry.size = index.readUInt32BE(p)
  p += 4
  entry.sha1 = index.slice(p, p + 20)
  p += 20
  entry.flags = index.readUInt16BE(p)
  p += 2
  entry.assumeValid = entry.flags & 0x8000
  entry.extended = entry.flags & 0x4000
  entry.stage =  [entry.flags & 0x2000, entry.flags & 0x1000]
  entry.nameLen = entry.flags & 0xFFF
  entry.length = 62

  if (entry.extended && result.version === 3) {
    entry.extraFlags = index.readUInt16BE(p)
    p += 2

    entry.reserved = entry.extraFlags & 0x8000
    entry.skipWorktree = entry.extraFlags & 0x4000
    entry.intentToAdd = entry.extraFlags & 0x2000
    entry.length += 2
  }

  if (entry.nameLen < 0xFFF) {
    entry.name = index.slice(p, p + entry.nameLen).toString('utf8')
    p += entry.nameLen
    entry.length += entry.nameLen
  } else {
    const a = []
    do {
      a.push(index[p++])
    } while (index[p] !== 0)
    entry.nameLen = a.length
    entry.length += entry.nameLen + 1
    entry.name = Buffer.from(a).toString('utf8')
  }

  const padLen = (8 - entry.length % 8) || 8
  const pad = index.slice(p, p + padLen)
  p += padLen
  for (let i = 0; i < padLen; i++) {
    if (pad[i] !== 0) {
      throw new Error('non-null chars in pad')
    }
  }
}
console.log(result)