How can I tell typescript that an object's function will populate an otherwise null property?

My problem is similar to this:

Type Name = {firstName: string, lastName: string}

class NameClass {
  public _name: Name | null;

  constructor() {
    this._name = null;
  }

  // or any function
  public set name(name: Name) {
    this._name = name;
  }

  public printName() {
    if (!this._name) this.name = { firstName: "should not", lastName: "be null" };
    //          v Object may be null even though I just set it ^
    console.log(this._name.firstName);
  }
}

I'm aware I can simply use the " ! " assertion to get around this or can create a separate function to make sure that the object is not null, but I'm looking for a way to tell typescript that the function will set the property.

There is a solution proposed in question

but I find this kind of hacky and doesn't really solve the problem. The post was also 4 years ago, so I'm hoping there is a better way to fix this?

Have you considered initializing the _name property in the constructor?

type Name = {firstName: string, lastName: string}

class NameClass {
  public _name: Name;

  constructor() {
    this._name = { firstName: "should not", lastName: "be null" };
  }

  // or any function
  public set name(name: Name) {
    this._name = name;
  }

  public printName() {
    console.log(this._name.firstName);
  }
}

Typescript is okay with that, because that way _name is always set.

If im not mistaken, the problem is not telling typescript "how to do it" - it is simply not possible. The class cannot and will never know, what nullable property was set in what function (or some other way). The whole point of having a nullable property is to have the opportunity to set that property to null. Even after setting the property, it may return back to null (e. g. through another function).

A work around would be having that property set to defalt string values ("") in the constructor.

Also as a side-note, it is not recommendet to have a public property start with an underscore. Normally, only private fields are named like that.

This version type checks:

type Name = { firstName: string; lastName: string };

class NameClass {
  public _name: Name | null;

  constructor() {
    this._name = null;
  }

  ensureName(): asserts this is { _name: Name } {
    if (!this._name)
      this.name = { firstName: 'should not', lastName: 'be null' };
  }

  // or any function
  public set name(name: Name) {
    this._name = name;
  }

  public printName() {
    this.ensureName();
    
    console.log(this._name.firstName);
  }
}

but I think using the non-null assertion operator is more idiomatic, and doesn't give you less guarantees, as TS mostly trusts you regarding the asserts ... syntax.