Type guards

Can be used as a good alternative to as.

const user = data as User;

function isUser(obj: any): obj is User {
  return typeof obj === 'object' &&
         obj !== null &&
         typeof obj.name === 'string' &&
         typeof obj.age === 'number';
}
 
if (isUser(data)) {
  // TypeScript now knows `data` is a User
  console.log(data.name);
}