Collection of typescript type system issues or bugs that don't get caught by it.

first-one.ts

type Foo = {
  // Assigning undefined here is wrong.
  // this value either HAVE A KEY WITH VALUE or DON'T HAVE A KEY
  [key: string]: { foo: string }
  // const foo: Foo = { key: undefined }
  // 'key' in foo // true, but should never be the case, because if there's a key, there's a value.
}

// note: the only case where the index signature might be correctly represented
// is when we use a proxy, that way we can guarantee that we always return something 
// for any index access


const fooRecord: Foo = {
  // Hey, why ts is not giving me an error here? why is it not requiring me
  // to implement all the infinite sequences on this dict?
  "foo": { foo: '' }
}

function bar(foo: Foo) {
  // foo["bar"] is recognized as never returning undefined, so the || is basically dead code for ts type system
  // but we receive no warning about that.. this is actually very dangerous
  const wrongType = foo["bar"] || 121;

  wrongType.foo.trim() // error :DDDDDDD
}

bar(fooRecord)