Not sure if this is a place to ask this but if someone does not have experience working with Javascript, they might have trouble reasoning about this code:

https://codesandbox.io/s/is849

edit: complete code

```typescript

class Person { id: number; name: string; yearOfBirth: number;

    constructor(id: number, name: string, yearOfBirth: number) {
        this.id  = id;
        this.name = name;
        if (yearOfBirth < 1900 || yearOfBirth > 2020) {
            throw new Error("I don't understand you. Go back to your time machine.");
        } else {
            this.yearOfBirth = yearOfBirth;
        }
    }

    getAge(): number {
        const currentDate: number = new Date().getUTCFullYear();
        return currentDate - this.yearOfBirth;
    }

}

class Dog { id: number; name: string; yearOfBirth: number;

    constructor(id: number, name: string, yearOfBirth: number) {
        this.id  = id;
        this.name = name;
        if (yearOfBirth < 1947 || yearOfBirth > 2020) {
            throw new Error("I don't understand you. Go back to your time machine.");
        } else {
            this.yearOfBirth = yearOfBirth;
        }
    }

    getAge(): number {
        const currentDate: number = new Date().getUTCFullYear();
        return (currentDate - this.yearOfBirth) * 7;
    }

}

const buzz: Person = new Person(1, `Buzz`, 1987);

const airbud: Dog = buzz;

console.log(`Buzz is ${buzz.getAge()} years old.`);

console.log(`Airbud is ${airbud.getAge()} years old in human years.`);

```

Unfortunately HN doesn't support commonmark's triple backtick code blocks. You'll need to use 4 spaces before each line of code.

Two spaces. Four will work, of course, it just wastes a bit of horizontal screen space.

What’s confusing? I must’ve missed it in my cursory scan.

Dog and Person are structurally the same so you can assign a person to a dog and vice versa. But that's just how structural typing works and as a user of TypeScript I haven't run into a case where this'd be an issue.

Haha oh. Yeah, assumed that the Dog definition wasn’t worthless. Indeed TS is structurally typed. And that’s nice!

I worked on a web based editor. A library would give us a range to highlight, in 1-based coordinates. The editor control was 0-based. As you can imagine it was easy to forget to translate back and forth in one path or another. In a strongly typed language I would simply define two Range types and the compiler would eliminate the mistake. I assumed Typescript could help me in the same way but it allowed the two types to be interchanged silently because they had the same structure. Perhaps I was holding it wrong?

Typescript has two hacks that help with mixing of similar data and introduce somewhat-nominal typing - branding and flavoring [1]. Also see smart constructors [2] for more functional approach.

[1] https://gist.github.com/dcolthorp/aa21cf87d847ae9942106435bf...

[2] https://dev.to/gcanti/functional-design-smart-constructors-1...

That's the most they could simplify the code to make the point?

A person value assigned to a dog variable