Stable JSON stringify

1. stableJSONStringify.ts

// Make sure JSON keys are always in the same order, to allow for easy comparison
// This uses JSON.stringify's replacer twice:
// - once to get the list of keys, including of nested children
// - once in its lesser known form: as a array of keys
//     this filters keys to only ones in the list - (we don't use this feature)
//     but also sorts them, and this is why this is useful to us
export function stableJSONStringify(obj: any) {
  const allKeys: string[] = [];

  // Use the replacer function to get the keys
  JSON.stringify(obj, (k, v) => {
    if (k !== '') allKeys.push(k);
    return v;
  });

  // Sorted and unique list of keys
  const uniqueKeys = allKeys.filter((k, i) => allKeys.indexOf(k) === i);
  const sortedKeys = uniqueKeys.sort();

  // Use the "filter" behavior (that also sorts keys in the order provided)
  // of the replacer parameter of JSON.stringify to ensure a stable stringification
  return JSON.stringify(obj, sortedKeys);
}

2. stableJSONStringify.test.ts

import { stableJSONStringify } from './stableJSONStringify';

// Numbers are used as values and are used to represent the order they should be showing up in the output

describe('stableJSONStringify', () => {
  it('should sort keys', () => {
    const obj = {
      b: 2,
      a: 1,
    };
    expect(stableJSONStringify(obj)).toMatchSnapshot();
  });

  it('should sort nested keys', () => {
    const obj = {
      b: 3,
      a: {
        d: 2,
        c: 1,
      },
    };
    expect(stableJSONStringify(obj)).toMatchSnapshot();
  });

  it('should sort nested keys in an array', () => {
    const obj = {
      b: 3,
      a: [
        {
          d: 2,
          c: 1,
        },
      ],
    };
    expect(stableJSONStringify(obj)).toMatchSnapshot();
  });
});

3. stableJSONStringify.test.ts.snap

// Jest Snapshot v1, https://goo.gl/fbAQLP

exports[`stableJSONStringify should sort keys 1`] = `"{\\"a\\":1,\\"b\\":2}"`;

exports[`stableJSONStringify should sort nested keys 1`] = `"{\\"a\\":{\\"c\\":1,\\"d\\":2},\\"b\\":3}"`;

exports[`stableJSONStringify should sort nested keys in an array 1`] = `"{\\"a\\":[{\\"c\\":1,\\"d\\":2}],\\"b\\":3}"`;