1. export function cloneElement(element, config, children) {
    2.   if (element === null || element === undefined) {
    3.     throw new Error(
    4.       `React.cloneElement(...): The argument must be a React element, but you passed ${element}.`,
    5.     );
    6.   }
    8.   let propName;
    10.   // Original props are copied
    11.   const props = Object.assign({}, element.props);
    13.   // Reserved names are extracted
    14.   let key = element.key;
    15.   let ref = element.ref;
    16.   // Self is preserved since the owner is preserved.
    17.   const self = element._self;
    18.   // Source is preserved since cloneElement is unlikely to be targeted by a
    19.   // transpiler, and the original source is probably a better indicator of the
    20.   // true owner.
    21.   const source = element._source;
    23.   // Owner will be preserved, unless ref is overridden
    24.   let owner = element._owner;
    26.   if (config != null) {
    27.     if (hasValidRef(config)) {
    28.       // Silently steal the ref from the parent.
    29.       ref = config.ref;
    30.       owner = ReactCurrentOwner.current;
    31.     }
    32.     if (hasValidKey(config)) {
    33.       key = '' + config.key;
    34.     }
    36.     // Remaining properties override existing props
    37.     let defaultProps;
    38.     if (element.type && element.type.defaultProps) {
    39.       defaultProps = element.type.defaultProps;
    40.     }
    41.     for (propName in config) {
    42.       if (
    43.         hasOwnProperty.call(config, propName) &&
    44.         !RESERVED_PROPS.hasOwnProperty(propName)
    45.       ) {
    46.         if (config[propName] === undefined && defaultProps !== undefined) {
    47.           // Resolve default props
    48.           props[propName] = defaultProps[propName];
    49.         } else {
    50.           props[propName] = config[propName];
    51.         }
    52.       }
    53.     }
    54.   }
    56.   // Children can be more than one argument, and those are transferred onto
    57.   // the newly allocated props object.
    58.   const childrenLength = arguments.length - 2;
    59.   if (childrenLength === 1) {
    60.     props.children = children;
    61.   } else if (childrenLength > 1) {
    62.     const childArray = Array(childrenLength);
    63.     for (let i = 0; i < childrenLength; i++) {
    64.       childArray[i] = arguments[i + 2];
    65.     }
    66.     props.children = childArray;
    67.   }
    69.   return ReactElement(element.type, key, ref, self, source, owner, props);
    70. }