Support readonly types with inference

[orta]

Search Terms

readonly without type

Suggestion

Today, in order to use a readonly type with inference you need to know typeof exists

const user = {
  name: "orta"
}

const readonlyUsers: readonly typeof user[] = [user] 

It would be nice to support shorthand syntax for this:

const user = {
  name: "orta"
}

const readonlyUsers: readonly = [user] 

Where the lack of a definition outside of readonly implies you want to keep the inferred type.

Use Cases

Making it cheaper syntactically to use readonly more often without using slightly more esoteric typescript features.

Could be negated by these though:

• Immutable-By-Default Flags #32758
• Default to ReadonlyArray #32467

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. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[RyanCavanaugh]

Can you compare and contrast with how as const doesn't address this use case?

[jack-williams]

From the PR that added readonly (#29435) there was a question about the behaviour of types called readonly here. The following used to compile, but does not currently:

type readonly = string;
const q: readonly = 'hello';

I think there was a plan to fix this, but maybe though would need to be reviewed in light of this proposal.

[orta]

Hrm great point - I actually can't think of any reason why as const doesn't cover this other than for the exact case I was looking at (which was to show a particular error message )

I'm fine with this being considered closed 👍

[RyanCavanaugh]

As an aside I think we're happy with readonly being (unintentionally?) reserved for the time being - who knows what we might want with it later.