代码：get(key: string){// return 'this is a service';return JSON.parse(localStorage.getItem(key));}编译错误：
(method) Storage.getItem(key: string): string | null
Returns the current value associated with the given key, or null if the given key does not exist in the list associated with the object.Argument of type 'string | null' is not assignable to parameter of type 'string'.Type 'null' is not assignable to type 'string'.ts(2345)
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修改如下调试通过：

  get(key: string){// return 'this is a service';return JSON.parse(localStorage.getItem(key) || '{}');}