Casting to Partial<x> seems to be errorful

I have this:

export interface HavenData {
  timeoutAmount: number;
  throwSync?: boolean;
  timeoutThrow: boolean;
  promiseThrow: boolean;
}

 const qs = {timeout: Math.ceil(30 * Math.random())} as Partial<HavenData>;

shouldn't that be an error since timeout is not a member of the HavenData interface? I am not seeing a problem in my IDE.

Type assertions are meant to be as lenient as possible, so excess property errors won't occur; that means you're almost always able to use a type assertion with something that's all-optional.

Ryan Cavanaugh (@RyanCavanaugh)

I feel like Ryan Cavanaugh (@RyanCavanaugh) might have opinions about this though.

I was thinking that this was like "F you, just do what I want":

const qs = <Partial<HavenData>>{timeout: Math.ceil(30 * Math.random())};

but that this was more like "please complain if the cast will likely not work":

const qs = {timeout: Math.ceil(30 * Math.random())} as Partial<HavenData>;

Nope, the two are semantically equivalent; x as Foo was introduced because of parsing problems related to JSX, but it works the same in the type system.

If you want the compiler to complain, I'd go for a type annotation instead of an assertion:

const qs: Partial<HavenData> = { timeout: Math.ceil(30 * Math.random()) };
//                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// Type '{ timeout: number; }' is not assignable to type 'Partial<HavenData>'.
// Object literal may only specify known properties, and 'timeout' does not exist 
// in type 'Partial<HavenData>'.

Daniel Rosenwasser (@DanielRosenwasser)

I'm vaguely surprised this isn't an error due to Partial<T> being a weak type. Did we carve out something specific that makes this work?

Daniel Rosenwasser (@DanielRosenwasser) What is the proposal here?

Daniel Rosenwasser (@DanielRosenwasser)

Daniel Rosenwasser (@DanielRosenwasser) ping on the above question

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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Casting to Partial<x> seems to be errorful #22806

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Casting to Partial<x> seems to be errorful #22806

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DanielRosenwasser commented on Mar 22, 2018

DanielRosenwasser commented on Mar 22, 2018

ORESoftware commented on Mar 22, 2018

DanielRosenwasser commented on Mar 22, 2018

jcalz commented on Mar 23, 2018

RyanCavanaugh commented on Mar 23, 2018

RyanCavanaugh commented on Apr 16, 2018

typescript-bot commented on Dec 15, 2018

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