Establish a prototype attribute

Question

Establish a prototype attribute

From what I understand, TypeScript does not make a distinction between prototype properties and instance properties. Additionally, there is no built-in detection for Object.defineProperty on the prototype, which is unlike a feature for typechecking JavaScript.

Therefore, is the conventional method to create a prototype property simply by declaring it and defining it outside the class, without TypeScript being able to verify this?

class C {
  declare readonly x: string;
}
Object.defineProperty(
  C.prototype,
  'x',
  { value: 'something' },
);

In my specific scenario, I am extending the Error class and aim to override Error.prototype.name. It appears that the most effective approach is to add a prototype property on the subclass.

typescript

Answer 1

Answer №1

I believe the most common practice is to utilize instance properties over prototype properties in Typescript. Prototype properties are not widely supported and do not align well with idiomatic Typescript.

class A {
    // using instance property instead of prototype property
    readonly x: string = 'foo';
}

If memory efficiency is a concern, especially when dealing with numerous instances of A, you can define a property accessor using the get keyword. This accessor is associated with the class rather than individual instances:

class B {
    // acts like a readonly property but tied to the prototype
    get x(): string {
        return 'foo';
    }
}
// Outputs "foo"
console.log(new B().x);
// Returns false
console.log(new B().hasOwnProperty('x'));
// Returns true
console.log(B.prototype.hasOwnProperty('x'));

If there is a worry about the negligible performance impact of an accessor method compared to a simple property, then the restriction on assigning C.prototype.x = ...; directly is due to declaring it as readonly. By making it non-readonly, you can enable type-checking without restrictions on prototype assignments:

class C {
    // not marked as readonly to allow direct assignment to the prototype
    declare x: string;
}
// No issue
C.prototype.x = 'foo';
// Error expected
C.prototype.x = { value: 'bar' };

Playground Link

Answer 2

I believe the most common practice is to utilize instance properties over prototype properties in Typescript. Prototype properties are not widely supported and do not align well with idiomatic Typescript.

class A {
    // using instance property instead of prototype property
    readonly x: string = 'foo';
}

If memory efficiency is a concern, especially when dealing with numerous instances of A, you can define a property accessor using the get keyword. This accessor is associated with the class rather than individual instances:

class B {
    // acts like a readonly property but tied to the prototype
    get x(): string {
        return 'foo';
    }
}
// Outputs "foo"
console.log(new B().x);
// Returns false
console.log(new B().hasOwnProperty('x'));
// Returns true
console.log(B.prototype.hasOwnProperty('x'));

If there is a worry about the negligible performance impact of an accessor method compared to a simple property, then the restriction on assigning C.prototype.x = ...; directly is due to declaring it as readonly. By making it non-readonly, you can enable type-checking without restrictions on prototype assignments:

class C {
    // not marked as readonly to allow direct assignment to the prototype
    declare x: string;
}
// No issue
C.prototype.x = 'foo';
// Error expected
C.prototype.x = { value: 'bar' };

Playground Link

Establish a prototype attribute

Answer №1

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