Hashable JSON

Object keys in a Javascript maintain insertion order.

For some crypto tasks, such as hashing and signing, it's desirable to have a normalized view of the data.

Fortunately it's easy enough to recursively sort object keys using just a few lines of code:

function replacer (key, value) {
  switch (Object.prototype.toString.call(value)) {
    
    // sort object keys lexicographically
    case '[object Object]':
      return Object.fromEntries(Object.entries(value).sort())
    
    default:
      return value
  }
}

Example

You can use it as you normally would with JSON.stringify():

const example = {
  "isbn": "123-456-222",
  "author": {
    "lastname": "Doe",
    "firstname": "Jane"
  },
  "editor": {
    "lastname": "Smith",
    "firstname": "Jane"
  },
  "title": "The Ultimate Database Study Guide",
  "category": [
    "Non-Fiction",
    "Technology"
  ]
}

> JSON.stringify(example, replacer, 2)

{
  "author": {
    "firstname": "Jane",
    "lastname": "Doe"
  },
  "category": [
    "Non-Fiction",
    "Technology"
  ],
  "editor": {
    "firstname": "Jane",
    "lastname": "Smith"
  },
  "isbn": "123-456-222",
  "title": "The Ultimate Database Study Guide"
}

Additional types

It's also possible to extend this method to other types:

function replacer (key, value) {
  switch (Object.prototype.toString.call(value)) {
    
    // sort object keys lexicographically
    case '[object Object]':
      return Object.fromEntries(Object.entries(value).sort())
    
    // Unicode Normalization Form KC
    case '[object String]':
      return value.normalize('NFKC')
    
    // round floats to 7 decimal places of precision
    case '[object Number]':
      return value % 1 === 0
        ? value
        : Math.round(parseFloat(value) * 1e7) / 1e7
    
    default:
      return value
  }
}