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Comment by dvlsg | Hacker News Reader

> I very rarely see TypeScript developers actually use `any` (usually to reflect something like something parsed from arbitrary JSON that needs to be interrogated rather than just type-asserted)

Even that example usage of `any` should be less common, now. I think `unknown` (which is fairly new) is better there.

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Is it? My impression is that `unknown` type doesn't solve that specific problem.

    const out: unknown = JSON.parse("{a: 1}");
    if (out && typeof out.a === 'number') console.log(out.a);

gives the error:

    [ts] Object is of type 'unknown'. [2571]

Pretty sure you’re missing a typeof check for ‘out’.

I'm pretty sure I'm not.

    const out: unknown = JSON.parse("{a: 1}");
    if (typeof out === 'object' && out && typeof out.a === 'number') {
      console.log(out.a);
    }

gives the error:

    [ts] Property 'a' does not exist on type 'object'. [2339]

It's a completely unnecessary check, anyway: `out &&` is sufficient to exclude `null` and `undefined`, which is all that's needed to ensure that `out.a` won't crash. I challenge you to give a single input to `JSON.parse` that would crash my code without the `typeof` check.

I see what you mean. I guess unknown just doesn't work that well with inline type checks for objects, although in that case you probably want a function anyways. Using the `in` return type you can do something like this:

  const out: unknown = JSON.parse("{a: 1}");
  
  interface MyStructure {
      a: number;
  }
  
  function isMyStructure(toCheck: unknown): toCheck is MyStructure {
      return toCheck instanceof Object && 'a' in toCheck;
  }
  
  if (isMyStructure(out)) {
      console.log(out.a);
  }

This will give you no type errors, and gives you the benefit of having a re-usable function for this validation. I just discovered this today. I seem to have missed it in a recent release note.

Oh, yeah, that feature's existed for years; it's called "user-defined type guards".

https://www.typescriptlang.org/docs/handbook/advanced-types....

Unfortunately, it doesn't work in `checkJs` mode, and I always wish I could do it inline.

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