When working with class hierarchies, it can be possible to get the functions which you override out of sync because prior to 4.3, there is no type-safe way to ensure a function is always overriding an existing function. For example, this Account class expects its one function to be overridden when a user is an admin:

class Account {
  doSomething() {
    console.log("Make me a sandwich");
  }
}

class Admin extends Account {
  doSomething() {
    console.log("Sudo make me a sandwich");
  }
}

// You can change the names of these functions, for example maybe 'doSomething' is a bit vague. If you change the name to 'performAction' in Account, but _not_ in Admin you have decoupled the functions un-expectedly. To enforce the consistency, there is a new flag: noImplicitOverride and additional syntax. To see it in action, delete the space after the @ in the following comment: @ noImplicitOverride Then you need to add `override` where the red error markers are.