每天学习一点底层逻辑 😂😂😂😂😂 😂😂😂😂😂 😂😂😂😂😂 😂😂😂😂😂 😂😂😂😂😂
// ATTENTION 注意
// When adding new symbols to this file,
// Please consider also adding to 'react-devtools-shared/src/backend/ReactSymbols'
// The Symbol used to tag the ReactElement-like types.
var REACT_ELEMENT_TYPE = Symbol.for('react.element');
var REACT_PORTAL_TYPE = Symbol.for('react.portal');
var REACT_FRAGMENT_TYPE = Symbol.for('react.fragment');
var REACT_STRICT_MODE_TYPE = Symbol.for('react.strict_mode');
var REACT_PROFILER_TYPE = Symbol.for('react.profiler');
var REACT_PROVIDER_TYPE = Symbol.for('react.provider');
var REACT_CONTEXT_TYPE = Symbol.for('react.context');
var REACT_FORWARD_REF_TYPE = Symbol.for('react.forward_ref');
var REACT_SUSPENSE_TYPE = Symbol.for('react.suspense');
var REACT_SUSPENSE_LIST_TYPE = Symbol.for('react.suspense_list');
var REACT_MEMO_TYPE = Symbol.for('react.memo');
var REACT_LAZY_TYPE = Symbol.for('react.lazy');
var REACT_OFFSCREEN_TYPE = Symbol.for('react.offscreen');
var MAYBE_ITERATOR_SYMBOL = Symbol.iterator;
var FAUX_ITERATOR_SYMBOL = '@@iterator';
$$typeof是一个Symbol类型的变量，这个变量可以防止XSS
React定义了一些全局的Symbol变量，利用 ES2016 Symbol 的特性来定义各种React类型以供框架使用 Symbol表示独一无二的值
