jsum

JSum

Consistent checksum calculation of JSON objects.

Quick start

const JSum = require('jsum')

const obj1 = {foo: [{c: 1}, {d: 2, e: 3}], bar: {a: 2, b: undefined}}
const obj2 = {bar: {b: undefined, a: 2}, foo: [{c: 1}, {e: 3, d: 2}]}

console.log(JSum.digest(obj1, 'SHA256', 'hex')) // 7514a2664dab82189b89d8250da9d0e1e6c95d3efaca6ffc25e5db42d7a7d053
console.log(JSum.digest(obj2, 'SHA256', 'hex')) // 7514a2664dab82189b89d8250da9d0e1e6c95d3efaca6ffc25e5db42d7a7d053

Why this module?

My main goal was to create Etags from JSON objects. The most intuitive approach would have been something like:

const crypto = require('crypto')

function checksum (obj) {
  return crypto.createHash('MD5').update(JSON.stringify(myObj)).digest('hex')
}

However, this approach would yield two different results for semantically same JSON objects:

console.log(checksum({"a": 1, "b": 2})) // 608de49a4600dbb5b173492759792e4a
console.log(checksum({"b": 2, "a": 1})) // 9915965eb40d343a8fe26e4e341d1a05

JSum on other hand makes sure that semantically same JSON objects always get the same checksum! Moreover, it provides a good deal of time advantage over some other viable modules*:

Module	Time (ms) to hash a 181 MB JSON file (from memory)
json-hash	81537
json-stable-stringify	12134
JSum	9656
json-checksum	FATAL ERROR: [...] - process out of memory

*NOTE: The measurements above are not from formal benchmarking. A huge random JSON file (181 MB) was taken as the base for benchmarking. The listed modules were used to create SHA256 hash of that file. To measure the time, internal console.time(() and console.timeEnd() methods were used.

I don't whant this :-(

Fair enough! Just copy (check the license first!) this for your own code and hash as you will:

/**
 * Stringifies a JSON object (not any randon JS object).
 *
 * It should be noted that JS objects can have members of
 * specific type (e.g. function), that are not supported
 * by JSON.
 *
 * @param {Object} obj JSON object
 * @returns {String} stringified JSON object.
 */
function serialize (obj) {
  if (Array.isArray(obj)) {
    return JSON.stringify(obj.map(i => serialize(i)))
  } else if (typeof obj === 'object' && obj !== null) {
    return Object.keys(obj)
      .sort()
      .map(k => `${k}:${serialize(obj[k])}`)
      .join('|')
  }

  return obj
}