I’ve been using Java generics for years, and when I saw their syntax in TypeScript, I simply put a checkmark in the list of TypeScript features that I already know and understand.I was wrong. Let me show you something.
Below is a Java code sample that illustrates the use of generics. I’ve created a class Person and its subclass Employee. Then I created a standalone class Animal. Finally, I used the generic notation to ensure that if anyone would try to add an instance of Animal to the collection of workers, the Java compiler would complain, and it did:
Then I re-wrote the same program in TypeScript, and its compiler didn’t complain:
But if I’ll comment out the property name in the class Animal, the TypeScript compiler will complain:
This may lead to the following conclusions:
1. In TypeScript, if you use a type as a parameter in generics, it’ll allow any other type as long as it has the same properties (e.g. Person.name and Animal.name)
2. In TypeScript, animals can be workers
If there is something in TypeScript documentation that has different explanations to this behavior, please let me know. Generics is an interesting subject and I’ll write another blog soon.
yakovfain.com 
I think it is not related to Generics. It has to do with the kind of type systems of the different languages: I mean, Java use a Nominal typing whereas Typescript use a structural typing.
Have a look a the different typing systems:
https://numberfour.github.io/n4js/features/nominal-and-structural-typing.html
Agree.
This code is also perfectly valid:
class Person {
name: string;
}
class Animal {
name: string;
}
let not_a_person: Person = new Animal();
It’s due to typescripts “duck typing”
Think of it like all a person needs to be a person is a name, and animals have a name parameter. When typescript expects a person and you give it anything that has the same parameters as a person, it gets silently cast to a person.
If you were to however like put a paremeter like say language on person, but not on animal, then there would be an error.
Possibly, such behavior is caused because of this:
>In TypeScript, two types are compatible if their internal structure is compatible.
https://www.typescriptlang.org/docs/tutorial.html
Just red this part of the tutorial, if mr. Yakov don’t notice I think I would easily forget until meet during coding. After long time work with Java we simply applying java rules to TypeScript.