Classes and Generics In TypeScript

Question

class Person {
  constructor(public name: string) {}
}

class Manager extends Person {}

class Admin extends Person {}

class School {
  constructor(public name: string) {}
}

function doOperation<T extends Person>(person: T): T {
  return person;
}

let person = doOperation(new Manager("tars"));
let school = doOperation(new School("Harper's"));

why does typescript not throw any error in this case, when School class is clearly not a subclass of Person. Is it because both the classes (Person and School) have a property with same name.

because they have the exact same properties, try changing name in school to schoolName ie public schoolName: string and it will throw an error. — tHeSiD
– tHeSiD, Commented May 29, 2021 at 18:45
Typescript uses a structural typing system, and not a nominal typing system. This means types that have the same interface, are considered the same type. — Alex Wayne
– Alex Wayne, Commented May 29, 2021 at 19:20

Countingstuff · Accepted Answer · 2021-05-29 19:15:53Z

Your example can be simplified to this being OK in typescript

class Person {
    constructor(public name: string) {}
}

class School {
    constructor(public name: string) {}
}

const m: Person = new School("schoolio");

As the tHeSiD says. This is how it is in typescript, the above is essentially a demonstration of what 'duck typing' is. A new School("schoolio") has all the properties needed to be a Person, so it can be used as one.

This is great, well I love it, because it means that class relationships don't 'get in the way'. But it is also sometimes annoying, because it means it's easy to call a function with a 'wrong type'

You just need to appreciate that what you lose in safety you gain in functions automatically being very generic; no longer are you constrained by having to pass things in of 'exactly the right type' - so long as you pass in objects which have the properties needed to run the function you are good to go.

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