[react] Consumer向上找不到Provider的时候怎么办？

并不会报错，取不到值而已

The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them. Note: passing undefined as a Provider value does not cause consuming components to use defaultValue.

找不到会取用默认值，注意provider的value设置为undefined不会覆盖默认值

当consumer向上层找不到provider时，此时就会取创建context时传给createContext的那个值，也就是当前context对象的默认值，在定义provider时的value中的值不是默认值，而是表示需要被传递的值