[DexesTTP]

TS allows you to pass a read-only object to a method taking a read-write value:

    type A = { value: number; }
    function test(a: A) { a.value = 3; }
    function main() {
      const a: Readonly<A> = { value: 1 };
      // a.value = 2; <= this errors out
      test(a); // this doesn't error out
      console.log(a); // shows 3
    }

[tossandthrow]

It seems super weird that this type checks, and it appears to be some sort of corner case explicitly implemented.

Normally typescript does not just allow implicit removal of a container type.

Like, you can't pass Array<A> to a function that just takes A.

[thoweofasdf]

Nice find. Never ran into this because I haven't mutated inputs in over 15 years.