Freshness broken for empty state

[basarat]

TypeScript Version: nightly

Code

class Foo extends React.Component<{}, {}>{
  render() { return <noscript /> }
  foo() {
    this.setState({ something: true }); // Should error but doesn't
  }
}

🌹

[normalser]

Isn't it because you can assign anything to {} ?

let a:{} = {something:true} // OK

FYI https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-all-types-assignable-to-empty-interfaces

[basarat]

It is and thats the issue I should have created.

But is there a reason why {} should be allowed to bypass the strict object checking (freshness) rule ? ❤️

[basarat]

@wallverb Thanks!

In general, you should never find yourself declaring an interface with no properties.

But we do cause setState is dangerous for beginners https://medium.com/@mweststrate/3-reasons-why-i-stopped-using-react-setstate-ab73fc67a42e#.ja8gt8txu so we swapped state to {}

And then migrating code from setState to @observable becomes painful to clean up as all those calls to setState don't highlight as errors. We instead needed to painfully find references to setState 🌹

[normalser]

@basarat Maybe exact types would help #12936

Here is the result in Flow:

type Foo = {  };
type Bar = {|  |};

const foo:Foo = { a: 4 } // Works
const bar:Bar = { a: 5 } // Error Property `a` not found in object type

or use never as State type in your React component

[basarat]

They would help.

I would still argue that it is okay that {foo:true} works with {}, but should respect the freshness check just like other things that fail in the presence of freshness 🌹

[RyanCavanaugh]

{ } is a frequent "safer" alternative to any for input positions.

Freshness detection is generally intended to detect typos (this.setState({ weigt: 12 });); the "typo" of including a property where none are allowed (?) is a very strange error to make. It's very unlikely the recipient of the value demands an empty object -- why would anyone be doing that?

[basarat]

why would anyone be doing that

@RyanCavanaugh for migrating off of setState : #13423 (comment) 🌹

[RyanCavanaugh]

If you want something that nothing is assignable to, use void

[basarat]

If you want something that nothing is assignable to, use void

Excellent. That solves my need. Closing as I don't need this changed anymore 🌹 and the decision to do the freshness checks for {} is a choice I leave for you / other humans 🤗 , create an issue or ignore ❤️