error TS2564: Property ‘key’ has no initializer and is not definitely assigned in the constructor.
Info: This error occurs with TypeScript 2.7 in “strict” mode because TypeScript 2.7 introduced a new flag called
--strictPropertyInitialization, which tells the compiler to check that each instance property of a class gets initialized in the constructor body, or by a property initializer. See Strict Class Initialization.
Bad Code ❌
Fixed Code ✔️