stableStringify.js

// @flow strict

import isObject from './isObject';

// Inspiration (modified and fixed stability):
// https://github.com/facebook/relay/blob/87930e12ed9694865d3a70dda564c0711e2890fd/packages/relay-runtime/util/stableCopy.js

/**
 * @see http://www.ecma-international.org/ecma-262/6.0/#sec-sortcompare
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort
 */
function defaultCompare(x, y): 0 | 1 | -1 {
  if (x === undefined && y === undefined) {
    return 0;
  } else if (x === undefined) {
    return 1;
  } else if (y === undefined) {
    return -1;
  }
  const xString = toString(x);
  const yString = toString(y);
  if (xString < yString) {
    return -1;
  } else if (xString > yString) {
    return 1;
  }
  return 0;
}

/**
 * @see http://www.ecma-international.org/ecma-262/6.0/#sec-tostring
 * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString
 */
function toString(obj): string {
  if (obj === null) {
    return 'null';
  } else if (typeof obj === 'boolean' || typeof obj === 'number') {
    return obj.toString();
  } else if (typeof obj === 'string') {
    return obj;
  } else if (typeof obj === 'symbol') {
    throw new TypeError();
  }
  // only Array and Object should be left
  return Object.entries(obj)
    .sort()
    .toString();
}

/**
 * Creates a copy of the provided value, ensuring any nested objects have their
 * keys sorted such that equivalent values would have identical JSON.stringify
 * results.
 *
 * Please note! This function sorts arrays as well which is the biggest
 * difference compared to any other solution. This means that it is truly stable
 * however it can be only used for stable stringification since order of elements
 * in arrays is otherwise significant!
 */
function stableSort(value: mixed) {
  if (Array.isArray(value)) {
    return value.map(stableSort).sort(defaultCompare);
  }
  if (!isObject(value)) {
    return value;
  }
  const keys = Object.keys(value).sort();
  const stable = {};
  for (let i = 0; i < keys.length; i++) {
    stable[keys[i]] = stableSort(value[keys[i]]);
  }
  return stable;
}

export default function stableStringify(
  value: mixed,
  replacer?: ?Array<string>,
  space?: number,
): string {
  return JSON.stringify(stableSort(value), replacer, space) ?? 'undefined';
}