Create-react-app: Unsupported `as const` TypeScript syntax

[mauricekleine]

Is this a bug report?

Yes.

Typescript 3.4 introduces as const syntax. However, using create-react-app with --typescript doesn't support this syntax yet, because it uses an older version of @babel/preset-typescript that doesn't support as const. PR https://github.com/facebook/create-react-app/commit/3c9c21a2858a3397cceca51b3a299878b29c5c7b updates this dependency, amongst others, which fixes this issue but it hasn't been released yet.

Steps to Reproduce

1. npx create-react-app my-app etc.
2. add some variable to app.tsx that uses as const
3. App crashes: unexpected token

Alternatively, following the same steps directly from the create-react-app source code doesn't result in the app crashing.

[ianschmitz]

The PR you referenced is included as part of the 3.0 alpha release. If you want to try it out see #6475. Otherwise it will be part of the 3.0 official release.

[mauricekleine]

Thanks for the clarification!

[TomasHubelbauer]

The same is true for readonly on array return types in case anyone else finds this issue looking for that.
I tried upgrading to the CRA3 release and upgrading TypeScript in the solution and it worked.