Linter objects to "any" even in type guards

[lcrocker]

The linter (correctly, in general) objects to any use of "any", but there's one place where TypeScript all but requires it, which is user-defined type guards. For example:

function isMyType(obj: any) obj is MyType {
    if (! obj) return false;
    if (! (obj instanceof Object)) return false;
    if (! ('number' === typeof obj.count)) return false;
    return true;
}

One might argue that this is an issue with TypeScript in that it doesn't allow the use of "unknown" here without throwing errors and doesn't have any other way to check for the existence and type of properties of an unknown. But until this is fixed in TypeScript, it would be handy if the linter made an exception for type guards.

[Soremwar]

You can absolutely use unknown here, in fact I'd say this is a perfect use case for unknown, since it will force you to assert the type and properties of the unknown value before accessing them, handling any possible runtime error

As for the use of any, your code as it is totally a thing you can do in Deno, but we have to make a distinction between implicit and explicit any

In the case of implicit any, TypeScript can't guess the type of a variable and in Deno settings this is listed as an error that won't allow you to run your code.

function x(y){} // TypeScript can't guess what "y" should be, throws an error

Explicit any on the other hand will allow you to run your code, but will be displayed as a warning when using the linter.

function x(y: any){} // Totally valid and runnable code, it will generate a warning for "any"

So this isn't so much about the behavior of any or unknown, but the lack of understanding of how they should be used

[kitsonk]

Correct, unknown should be used in user-defined type guards.

[lcrocker]

Using "unknown" in user-defined type guards is the right thing to do, but it doesn't work, at least in the current state of TypeScript because there's no way to assert the presence of a property.
See, e.g., microsoft/TypeScript#25720 , microsoft/TypeScript#21732 . We should say explicitly that we won't fix this because we expect TypeScript to fix their end.

[Soremwar]

@lcrocker

there's no way to assert the presence of a property

Read the docs man

This is a safe way of asserting a value using unknown, it won't trigger getters or return false on validation over falsy values

function checkProperty(value: unknown){
  if(typeof value === "object" && value !== null){
    return "my_property" in value;
  }
  return false;
}