Static inheritance behavior is surprising

[jvilk]

I recently learned that TypeScript classes inherit static methods and fields, which really surprised me! Unlike object instances, static properties aren't set in a constructor, so their static initialization can't be re-run for subclasses. As a result, inherited static fields in subclasses will take on the current value of their parents' field as their initial value, which can be surprising.

class A {
    static bar: string = "foo";
}

A.bar = "Gotcha";

class B extends A {
}

console.log(B.bar); // prints 'Gotcha'

Is this desired behavior?

[danquirk]

See some previous discussions about this:

https://typescript.codeplex.com/workitem/2047
#613

[jvilk]

I see. From a discussion on esdicuss, it looks like B.__proto__ = A, and thus B would inherit bar from A.

In other words, accessing B.bar would initiate a prototype property lookup that is found on A. Further updates to A.bar would be reflected in a read of B.bar. However, that means that in ES6, assigning a value to B.bar would change the shape of B, since B itself does not have that property defined!

TypeScript does not emulate this behavior, as the __extend function redefines bar on B, causing the two fields to be completely distinct.

I have no proposal to remedy the situation if you desire to keep static field/method inheritance, since __proto__ is nonstandard and is not present in legacy browsers. So I recognize the difficulty in addressing the situation in a backwards-compatible manner without overly complicating runtime boilerplate.