replace implicit 'any' with 'unknown'

Search Terms

noImplicitAny, strict, any, unknown

Suggestion

When enabling "strict": true or "noImplicitAny": true in a project, could we assign unknown to un-inferred/un-typed variables, instead of causing a compiler error? This would be a non-breaking change, since all cases where this would have an effect are already compiler errors.

Use Cases

It would allow avoiding having to specify a type when it’s being type-checked anyway. The current implementation forces converted-from-js projects to deal with a lot of compiler errors from the outset (many of which use various runtime type-checking methods so should be happy with implicit unknowns).

Examples

const getMessage = input => typeof input === 'string' ? input : 'hello';
const shortenMessage = message => message.substring(3);

the getMessage example would be an error currently, but would be fine in the new world and the function would have return type string. the shortenMessage example would have a changed error. instead of Parameter 'message' implicitly has an 'any' type we’d see an error at message.substring of Object is of type 'unknown'. To me that’s more intuitive in terms of what the problem actually is. message is unknown so it should have type unknown.

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript / JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. new expression-level syntax)

[RyanCavanaugh]

This would be a non-breaking change, since all cases where this would have an effect are already compiler errors.

In principle this is true, but I suspect there’s a lot of code like this where this change would introduce a giant wall of new errors:

// @ts-ignore
function fn(x, y, a, b, c, d) {
  // do tons of stuff with x y a b c and d
}

We could potentially have some implicitUnknown flag instead though

[nickmccurdy]

I had a discussion recently where I realized it would be useful to replace any with unknown in situations like this, as it’s a much safer value to work with and mirrors some of the recent changes made to type inference in generics.

However, a potential flaw in this suggestion is that if a type isn’t checked in the same project, the author may forget to add a type that should be added explicitly when it instead returns unknown. For example, the user may be writing a library function that gets JSON from an API using (await fetch(...)).json(), which returns Promise<any>. This option would return Promise<unknown> instead of encouraging the user to explicitly add a type, which could cause type errors or unhelpful development experience from code using the library’s return type in TypeScript.

As a result, it would be safer to add an implicitUnknown option that changes implicit type generation to use unknown instead of any (similar to what generic parameters already do) so that TypeScript users have the option of keeping noImplicitAny on. I think this could even be a good option to turn on by default, if it could be done in a way that didn’t cause extra compatibility type errors.