Contextually type class property initializers by extends/implements types

When a property has exactly one type from an extends or implements clause, we should contextually type that property's initializer by the type from the clause. For example:

interface ThingListener {
    handleEvent: (x: MouseEvent) => void;
}

class Foo implements ThingListener {
    handleEvent = x => {
        // No error, but this code is incorrect
        console.log(x.timestamp);
    }
}

Ideally we could take this one step further when widening. The following behavior is also undesirable:

interface HasLength {
    length: number
}

class Foo implements HasLength {
    // length: any, not really what we intended
    length = undefined;
}

var x = new Foo();
x.length = 'wat'; // should not be allowed

e.g. #3666

Is this not the same as #1373?

#1373 is about methods, this is about properties

So is this a broader proposal than #1373? In other words, in the following example, is the arrow function in SortByQueryId contextually typed?

Additionally, if we declared getvalue in ISortInfo as a property but defined it as a method in the class, would its parameter's type be inferred?

    export interface ISortInfo
    {
        getvalue(x: QuerySummary): number | string;
        order: SortDir;
        ordercalc?: number;
    }

    class SortByQueryId implements ISortInfo
    {
        getvalue = x => x.QueryId;
        order = SortDir.Flip;
    }

I don't think we should distinguish between method-style and property-style declarations in the interface - both are an equally valid "source" in terms of type information, regardless of how the class is implementing it.

In the example, I would expect the arrow function to be contextually typed under this proposal, but not #1373.

Noel Abrahams (@NoelAbrahams)

Agree with Noel Abrahams (@NoelAbrahams) - can we merge this with #1373? If this subsumes that proposal, can we dupe that one to this one?

Also merge with #3804 ? Then we include interfaces too.

👍

From #5181 the following sample reproduces this issue:

declare module Backbone {
    interface RoutesHash {
        [routePattern: string]: string | {(...urlParts: string[]): void};
    }
    class Events { }
    class Router extends Events {
        routes: RoutesHash;
    }
}

declare module Marionette {
    class AppRouter extends Backbone.Router {
    }
}

module Backbone.Tests {
    // Error, routes is not contextually typed
    class MyRouter extends Backbone.Router {
        routes = {
            "some/route": "someMethod",
            "some/otherRoute": () => {}
        };
    }
}

module Marionette.Tests {
    // Error, routes is not contextually typed
    class MyRouter extends Marionette.AppRouter {
        routes = {
            "some/route": "someMethod",
            "some/otherRoute": () => {}
        };
    }
}

Tentatively approved -- this would be a slight breaking change, but probably very much in the "good" direction (i.e. uncovers bugs that were going missed before). We will investigate breakages in our RWC suite to see what the net impact is.

I'd be fine with this breaking change.

Unfortunately, we couldn't come up with a solution that was both consistent and backward-compatible. The breaks in our Real World Code suite were more bad than good. See #6118 for details. I'm closing this for now.

Navigation Menu

Repository navigation

Contextually type class property initializers by `extends`/`implements` types #3667

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Contextually type class property initializers by extends/implements types #3667

Description

Activity

NoelAbrahams commented on Jun 29, 2015

RyanCavanaugh commented on Jun 29, 2015

DanielRosenwasser commented on Jun 29, 2015

RyanCavanaugh commented on Jun 29, 2015

sophiajt commented on Jul 16, 2015

tinganho commented on Jul 16, 2015

Gaelan commented on Sep 21, 2015

omidkrad commented on Oct 8, 2015

RyanCavanaugh commented on Nov 3, 2015

omidkrad commented on Nov 6, 2015

sandersn commented on May 6, 2016

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

Contextually type class property initializers by `extends`/`implements` types #3667