function mergeDefaults(objectValue, sourceValue) {
	      return objectValue === undefined ? sourceValue : merge(objectValue, sourceValue, mergeDefaults);
	    }
Used by `_.defaultsDeep` to customize its `_.merge` use.

@private
@param {*} objectValue The destination object property value.
@param {*} sourceValue The source object property value.
@returns {*} Returns the value to assign to the destination object.