How to provide default/fixed value for inheriting class?

[feliksik]

For simplicity, I will take the example from https://mobx-keystone.js.org/class-models/#inheritance:

@model("MyApp/Point")
class Point extends Model({
  x: prop<number>(),
  y: prop<number>(),
}) {
  get sum() {
    return this.x + this.y
  }
}

// note how `ExtendedModel` is used
@model("MyApp/Point3d")
class Point3d extends ExtendedModel(Point, {
  z: prop<number>(),
}) {

}

How can I make the Point3d have a fixed value of '12' for the x value (or at least define an optional default for the constructor)?

I guess I can make a factory method that takes only the required parameters sets it, like so:

@model("MyApp/Point3d")
class Point3d extends ExtendedModel(Point, {
  z: prop<number>(),
}) {
    static create(constructorParams: {y: number, z: number}){ // or constructorParams: Omit<ModelCreationData<Point>, "x"> & { z: number }
        return new Point3d(
            {
                ...constructorParams,
                x: 12,
            }
        )
    }
}

But this is a lot of type-specific boilerplate. Is there a more idiomatic way to do this?

[xaviergonz]

I just released v1.6.0, which removes the restriction of not being able to redeclare a property on an extended model. Note that it is now on the user side to make sure the base and the extended properties are compatible (basically kept the same). If care is not taken the type of the property might become "never", be wrong, or it might fail in runtime.

example:

@model("MyApp/Point")
class Point extends Model({
  x: prop(1),
  y: prop(2),
}) {}

@model("MyApp/Point3d")
class Point3d extends ExtendedModel(Point, {
  y: prop(100), // this overrides the default value
  z: prop(3),
}) {}

does that address your issue?

[feliksik]

Wow that's fast :-)
But I'm afraid is not working as I expected. E.g. if the parent has no defaults, but the subclass does have a default, the ModelCreationData type should become a bit looser; that is, I would expect to be able to omit y and z in the constructor data, as they have defaults; but Typescript does not like it:

@model("MyApp/Point")
class Point extends Model({
    x: prop<number>(),
    y: prop<number>(),
}) { }

@model("MyApp/Point3d")
class Point3d extends ExtendedModel(Point, {
    y: prop(100), // this overrides the default value
    z: prop(3),
}) { }

const p1: Point3d = new Point3d({
    x: 1, y: 2, z: 3 // ok 
})
const p2: Point3d = new Point3d({
    x: 1 // typescript complains that this does not fit the { x: number; y: number; } type of `Point`
})

Typescript says:

Argument of type '{ x: number; }' is not assignable to parameter of type '{ y?: number | null | undefined; z?: number | null | undefined; } & {} & { x?: number | undefined; y?: number | undefined; } & { x: number; y: number; }'.
  Property 'y' is missing in type '{ x: number; }' but required in type '{ x: number; y: number; }'.ts(2345)

BTW: thank you @xaviergonz , you have made a great tool!!!

[xaviergonz]

I'm afraid changing the type itself when inheriting does not get along well with typescript :-/

I don't know if you share lots of logic, but you could alternatively do this (not saying it is better than your current method though, it is more like a mixing):

const props = {
  x: prop<number>(),
  y: prop<number>(),
} as const

function getSum2d(this: Point) {
  return this.x + this.y
}

@model("MyApp/Point")
class Point extends Model(props) {
  getSum2d = getSum2d
}

@model("MyApp/Point3d")
class Point3d extends Model({
  ...props,
  y: prop(100), // this overrides the default value
  z: prop(3),
}) {
  getSum2d = getSum2d
}

const p1: Point3d = new Point3d({
  x: 1,
  y: 2,
  z: 3, // ok
})
const p2: Point3d = new Point3d({
  x: 1, // ok
})

[feliksik]

Aaah that's very interesting, thank you so much for your help Xavier! I'm rather new to the whole typescript ecosystem, so these kind of tricks did not occur to me right away.

I imagined there could be some super hacky magic with Omit on the parameters of ExtendedModel, but that's speculation, and in fact it's really not a big deal for me at the moment. Feel free to close this issue!