diff.js

iimport {
  isArray,
  isObject,
  isNull,
  isNil,
  isEqual,
  isUndefined,
  keys,
  uniq,
  each,
  times,
} from 'lodash';

function isSameType(a, b) {

  if (typeof a !== typeof b) {
    return false;
  }

  if (isArray(a) && isArray(b)) {
    return true;
  }

  if (isObject(a) && isArray(b)) {
    return false;
  }

  if (isArray(a) && isObject(b)) {
    return false;
  }

  if (isObject(a) && isNull(b)) {
    return false;
  }

  if (isNull(a) && isObject(b)) {
    return false;
  }

  return true;
}

function diff(a, b) {

  if (isEqual(a, b)) {
    return;
  }

  if (isUndefined(b)) {
    return null;
  }

  if (!isSameType(a, b)) {
    return b;
  }

  let _keys;
  let output;

  if (isArray(a)) {

    if (isNil(b)) {
      output = b;
    }

    const length = a.length >= b.length ? a.length : b.length;

    times(length, function(index) {

      const delta = diff(a[index], b[index]);

      output = output || [];

      if (isUndefined(delta)) {
        output.push(undefined);
      } else {
        output.push(delta);
      }

    });

  } else if (isObject(a)) {

    _keys = uniq(keys(a).concat(keys(b)));

    each(_keys, function(key) {

      const delta = diff(a[key], b[key]);

      if (delta) {
        output = output || {};
        output[key] = delta;
      }
    });

  } else {

    if (!isEqual(a, b)) {
      output = b;
    }
  }

  return output;
}