should error on assignment that shadows a prototype method

🔎 Search Terms

own property, shadow, prototype, method, field, inheritance

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20241201#code/MYGwhgzhAEBiD29oG8BQ0PWPAdhALgE4Cuw+8hAFAJQrqYP4AWAlhAHQAOh85+AnpwCmAWSHN4AE2gBeaDVkA+Og1WZseeCCHsQ8AOaUA5BCZhJ8AO4xuvXoKHQAtuKZSj1ANz01AXx-Q-gy2fA5iEpIKaGoYGhBaOnqGRi4R0LjQIfbCHt4M-r5AA

💻 Code

class Foo {
    constructor() {
        this.prototypeMethod = () => {
            console.log('shadows prototype method');
        }
    }
    prototypeMethod() {
        console.log('method on prototype');
    }
}

🙁 Actual behavior

no error

🙂 Expected behavior

error - something like "Type 'Foo' has no instance property named 'prototypeMethod'. If you meant to reassign the method 'prototypeMethod', it must be a function-valued instance property instead of a prototype method.".

I expect an error here because the code is effectively equivalent to

class Foo {
    prototypeMethod = () => {
        console.log('shadows prototype method');
    };
    prototypeMethod() {
        console.log('method on prototype');
    }
}

which is a TS error (see https://github.com/microsoft/TypeScript/issues/13141).

Furthermore, it breaks error reporting in subclasses. The following code correctly reports a TS error due to the extended class not being able to override an instance property with a method: (playground)

class Foo {
    instanceProperty = () => {
        console.log('function-valued instance property');
    }
}

class Bar extends Foo {
    // TS ERROR: Class 'Foo' defines instance member property 'instanceProperty', but extended class 'Bar' defines it as instance member function.(2425)
    instanceProperty() {
        console.log('overridden with method')
    }
}

const b = new Bar();
b.instanceProperty(); // prints 'function-valued instance property'

However, this does not (playground)

class Foo {
    constructor() {
        this.method = () => {
            console.log('function-valued instance property')
        }
    }
    method() {
        console.log('method');
    }

}

class Bar extends Foo {
    // no error here  :(
    method() {
        console.log('overridden with method')
    }
}

const b = new Bar();
b.method(); // prints 'function-valued instance property'

Additional information about the issue

inspired by https://github.com/typescript-eslint/typescript-eslint/issues/10427

cc @miguel-leon

microsoft / TypeScript