src/legacy/store/recycleNodesInto.js

/**
 * Copyright (c) 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule recycleNodesInto
 * @flow
 */

'use strict';

/**
 * Recycles subtrees from `prevData` by replacing equal subtrees in `nextData`.
 */
function recycleNodesInto<T>(prevData: T, nextData: T): T {
  if (prevData === nextData ||
      typeof prevData !== 'object' || !prevData ||
      typeof nextData !== 'object' || !nextData) {
    return nextData;
  }
  let canRecycle = false;

  // Assign local variables to preserve Flow type refinement.
  const prevArray = Array.isArray(prevData) ? prevData : null;
  const nextArray = Array.isArray(nextData) ? nextData : null;
  if (prevArray && nextArray) {
    canRecycle =
      nextArray.reduce((wasEqual, nextItem, ii) => {
        nextArray[ii] = recycleNodesInto(prevArray[ii], nextItem);
        return wasEqual && nextArray[ii] === prevArray[ii];
      }, true) &&
      prevArray.length === nextArray.length;
  } else if (!prevArray && !nextArray) {
    // Assign local variables to preserve Flow type refinement.
    const prevObject = prevData;
    const nextObject = nextData;
    const prevKeys = Object.keys(prevObject);
    const nextKeys = Object.keys(nextObject);
    canRecycle =
      nextKeys.reduce((wasEqual, key) => {
        const nextValue = nextObject[key];
        nextObject[key] = recycleNodesInto(prevObject[key], nextValue);
        return wasEqual && nextObject[key] === prevObject[key];
      }, true) &&
      prevKeys.length === nextKeys.length;
  }
  return canRecycle ? prevData : nextData;
}

module.exports = recycleNodesInto;