Svelte store and types

[lkj4]

Why do I not get an error message on the last line? The property kfjsdlf doesn't exist in test.

import { writable } from "svelte/store";

const test = {
  a: {
    b: 30,
    c: 20
  },
  d: true
};

const store = writable(test);

store.update((s) => ({ ...s, kfjsdlf: 0 }));

https://codesandbox.io/s/solitary-leftpad-wjh0py?file=/src/index.ts

[MilesWellsSVT]

If you check the type for the first argument of update you'll see it's type Updater<T> = (value: T) => T;

Since TS 3.5 the implicit type constraint for generics is unknown. This is why you can expand the type of the object you're returning.

AFAIK this has to do with the limitations of the TS compiler's ability to infer generic typings.