Add readonly to TypeScript type modifier

[tanhauhau]

Q	A
Fixed Issues?	Fixes #9503
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/10151/

[danez]

Thanks for all the amazing work 👍

[nicolo-ribaudo]

Maybe we should throw an error for https://github.com/Microsoft/TypeScript/pull/29435/files#diff-e119cc28df9582f9e1ca0d41720ea142R1026?

Unless this is valid, in which case we don't have enough info to know whether to throw or not.

type A = number[];
type B = readonly A;

[danez]

//cc @DanielRosenwasser ^ Above comment

[tanhauhau]

is there a TypeScript Playground that supports readonly?

[nicolo-ribaudo]

No, it hasn't been released yet.

[existentialism]

Maybe we should throw an error for Microsoft/TypeScript/pull/29435/files#diff-e119cc28df9582f9e1ca0d41720ea142R1026?

IMO yes, we should

[tanhauhau]

IMO yes, we should

Does that mean we throw a syntax error whenever readonly does not proceed with array literal or tuple literal?

[nicolo-ribaudo]

It depends on

Unless this is valid, in which case we don't have enough info to know whether to throw or not.

type A = number[];
type B = readonly A;

[nicolo-ribaudo]

@tanhauhau

Does that mean we throw a syntax error whenever readonly does not proceed with array literal or tuple literal?

Exactly, accordingly to https://github.com/Microsoft/TypeScript/blob/9d3707d671592d030386956c9ce39e539b8d0972/tests/baselines/reference/readonlyArraysAndTuples.errors.txt, we should throw.

[nicolo-ribaudo]

The circleci failure doesn't seem related to this PR 🤔

[nicolo-ribaudo]

Thanks!