react16 版本常见 api

本节目标：理解“react16 版本常见 api”的核心思路，并能把它用于实际开发或面试表达。

const React = {
  Children: {
    map,
    forEach,
    count,
    toArray,
    only
  },
  createRef,
  Component,
  PureComponent,
  createContext,
  forwardRef,
  Fragment: REACT_FRAGMENT_TYPE,
  StrictMode: REACT_STRICT_MODE_TYPE,
  unstable_AsyncMode: REACT_ASYNC_MODE_TYPE,
  unstable_Profiler: REACT_PROFILER_TYPE,
  createElement: __DEV__ ? createElementWithValidation : createElement,
  cloneElement: __DEV__ ? cloneElementWithValidation : cloneElement,
  createFactory: __DEV__ ? createFactoryWithValidation : createFactory,
  isValidElement: isValidElement,
  version: ReactVersion,
  __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: ReactSharedInternals
}