Classes & Interfaces — Series 3

Preview — 3 of 10 questions

javascript

interface Logger {
  prefix: string;
  log(message: string): void;
}

class ConsoleLogger implements Logger {
  prefix = "[app]";
}

ACompiles fine — a class only has to implement the properties, not the methods

BCompiles fine — log is treated as optional because it has no body in the interface

CCompile-time error — ConsoleLogger is missing the log method required by Logger

DCompile-time error — an interface may not mix properties and methods

javascript

class Session {
  userId?: string;
}

const s = new Session();
console.log(s.userId.length);

ACompile-time error — s.userId is string | undefined, so .length may not exist

BCompiles fine and logs 0

CCompiles fine and logs undefined

DCompile-time error — a class field may not be declared optional

javascript

interface HasId {
  id: string;
}

class Entity implements HasId {}

ACompiles fine — implements copies id into the class

BCompiles fine — id is inherited and initialized to undefined

CCompiles fine, but id is only available on instances typed as HasId

DCompile-time error — implements only checks the class; the members must still be declared on it

Answer all 10 questions (7 more), see explanations for every answer, and track your score.