lodash/internal/baseMerge.js

var arrayEach = require('./arrayEach'),
    baseForOwn = require('./baseForOwn'),
    baseMergeDeep = require('./baseMergeDeep'),
    isArray = require('../lang/isArray'),
    isLength = require('./isLength'),
    isObjectLike = require('./isObjectLike'),
    isTypedArray = require('../lang/isTypedArray');

/**
 * The base implementation of `_.merge` without support for argument juggling,
 * multiple sources, and `this` binding `customizer` functions.
 *
 * @private
 * @param {Object} object The destination object.
 * @param {Object} source The source object.
 * @param {Function} [customizer] The function to customize merging properties.
 * @param {Array} [stackA=[]] Tracks traversed source objects.
 * @param {Array} [stackB=[]] Associates values with source counterparts.
 * @returns {Object} Returns the destination object.
 */
function baseMerge(object, source, customizer, stackA, stackB) {
  var isSrcArr = isLength(source.length) && (isArray(source) || isTypedArray(source));

  (isSrcArr ? arrayEach : baseForOwn)(source, function(srcValue, key, source) {
    if (isObjectLike(srcValue)) {
      stackA || (stackA = []);
      stackB || (stackB = []);
      return baseMergeDeep(object, source, key, baseMerge, customizer, stackA, stackB);
    }
    var value = object[key],
        result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
        isCommon = typeof result == 'undefined';

    if (isCommon) {
      result = srcValue;
    }
    if ((isSrcArr || typeof result != 'undefined') &&
        (isCommon || (result === result ? result !== value : value === value))) {
      object[key] = result;
    }
  });
  return object;
}

module.exports = baseMerge;