Multidimensional readonly arrays

Search Terms

readonly array multi multidimensional

Suggestion

The readonly modifier should work with multidimensional arrays.

Use Cases

I assumed readonly number[][] to be multidimensional array which is readonly, but it translates to ReadonlyArray<number[]>, this feels like a footgun.

It is currently also not very intuitive which array is readonly: ReadonlyArray<number>[] or ReadonlyArray<number[]>, without trying it out.

Examples

I'd expect readonly number[][] to be equal to ReadonlyArray<ReadonlyArray<number>>

Explicitely "mixed mutable" arrays could be achieved using parens:

(readonly number[])[] == ReadonlyArray<number>[]
readonly (number[])[] == ReadonlyArray<number[]>

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.

Start by reviewing the proposed meanings for readonly number[][], ReadonlyArray<number>[], and ReadonlyArray<number[]>, along with the parenthesized alternatives. Determine whether the requested shorthand can be specified consistently without changing existing code, then document the accepted semantics and validate them against the listed examples.