Suggestion: Add an auto_init_properties flag, emitting error if default value cannot be determined

[mgenware]

This is related to #8476. I'd suggest that, under some flags, instead of warning user of type safety caused by the default undefined value, give it a default value like C#, if the default value can't be determined, emit an error.

// under a flag like `--auto_init_properties`
class Person {
  id: number; // auto initialized to 0
  name: string; // auto initialized to ""
  description: string|null; // auto initialized to null

  // **Compilation error: Can't determine default value for this prop, all properties must be initialized. **
  opt: string|number|Option; 

  // To fix this, user must specify an value, for example:
  opt: string|number|Option = 'abc';
  opt: string|number|Option = new Option();
}

In this way, all properties are actually defined in runtime:

const obj = new Person();
for (const property in obj) {
  console.log(property);
}

// Output: id, name, description, opt

Also, we don't need to write boilerplate code to enforce strict type safety:

class Person {
  constructor {
    this.id = 0;
    this.name = '';
    this.description = null;
    this.opt = new Option();
  }
}

[Ghabriel]

From the TypeScript Design Goals page, one of the non-goals of TS is to "Add or rely on run-time type information in programs, or emit different code based on the results of the type system," which I believe applies to your suggestion.