lodash/internal/baseMergeDeep.js

define(['./arrayCopy', '../lang/isArguments', '../lang/isArray', './isLength', '../lang/isPlainObject', '../lang/isTypedArray', '../lang/toPlainObject'], function(arrayCopy, isArguments, isArray, isLength, isPlainObject, isTypedArray, toPlainObject) {

  /** Used as a safe reference for `undefined` in pre-ES5 environments. */
  var undefined;

  /**
   * A specialized version of `baseMerge` for arrays and objects which performs
   * deep merges and tracks traversed objects enabling objects with circular
   * references to be merged.
   *
   * @private
   * @param {Object} object The destination object.
   * @param {Object} source The source object.
   * @param {string} key The key of the value to merge.
   * @param {Function} mergeFunc The function to merge values.
   * @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 {boolean} Returns `true` if the objects are equivalent, else `false`.
   */
  function baseMergeDeep(object, source, key, mergeFunc, customizer, stackA, stackB) {
    var length = stackA.length,
        srcValue = source[key];

    while (length--) {
      if (stackA[length] == srcValue) {
        object[key] = stackB[length];
        return;
      }
    }
    var value = object[key],
        result = customizer ? customizer(value, srcValue, key, object, source) : undefined,
        isCommon = typeof result == 'undefined';

    if (isCommon) {
      result = srcValue;
      if (isLength(srcValue.length) && (isArray(srcValue) || isTypedArray(srcValue))) {
        result = isArray(value)
          ? value
          : (value ? arrayCopy(value) : []);
      }
      else if (isPlainObject(srcValue) || isArguments(srcValue)) {
        result = isArguments(value)
          ? toPlainObject(value)
          : (isPlainObject(value) ? value : {});
      }
      else {
        isCommon = false;
      }
    }
    // Add the source value to the stack of traversed objects and associate
    // it with its merged value.
    stackA.push(srcValue);
    stackB.push(result);

    if (isCommon) {
      // Recursively merge objects and arrays (susceptible to call stack limits).
      object[key] = mergeFunc(result, srcValue, customizer, stackA, stackB);
    } else if (result === result ? (result !== value) : (value === value)) {
      object[key] = result;
    }
  }

  return baseMergeDeep;
});