Output a canonical normalised GeoJSON string which will hash deterministically

canonicalize.js

const geojson = {
  'id': -99,
  'type': -6,
  'properties': -5,
  'geometry': -4,
  'coordinates': -3,
  'bbox': -2
}

// GeoJSON Feature/Geometry
function isGeoJSON (path, ks) {
  return (
    (path === '$' && ['type', 'geometry'].every(k => ks.includes(k))) ||
    (path === '$.geometry' && ['type', 'coordinates'].every(k => ks.includes(k)))
  )
}

function sort (path, keys) {
  let sorted = keys.sort()

  // GeoJSON Feature/Geometry objects sort keys in custom order
  if (isGeoJSON(path, keys.map(s => s[0]))) {
    sorted = sorted.sort((a, b) => geojson[a[0]] - geojson[b[0]])
  }

  return sorted
}

function replacer (key, value, path) {
  switch (Object.prototype.toString.call(value)) {
    case '[object Object]': return Object.fromEntries(sort(path, Object.entries(value)))
    case '[object String]': return value.normalize('NFKC')
    case '[object Number]': return value % 1 === 0 ? value : Math.round(parseFloat(value) * 1e7) / 1e7
    default: return value
  }
}

function replacerWithPath (fn) {
  let paths = new Map()
  return function (key, value) {
    let path = (paths.get(this) || '$')
    if (key) path += Array.isArray(this) ? `[${key}]` : '.' + key
    let v = fn(key, value, path)
    if (v === Object(v)) paths.set(v, path)
    return v
  }
}

function squashArrays (str) {
  return str.replace(/\[[^\[\]]+\]/g, arr => {
    const squash = arr.replace(/\s\s+/g, ' ')
    return squash.length <= 60 ? squash : arr
  })
}

module.exports = (value) => squashArrays(JSON.stringify(value, replacerWithPath(replacer), 2))

// can be run directly
if (require.main === module) {
  const json = JSON.parse(require('fs').readFileSync(process.argv[2], 'utf8'))
  process.stdout.write(module.exports(json))
}