TypeScript transformer to convert readonly T[] to ReadonlyArray<T> in declaration files.

Starting from v3.4.0 (or one of the nightly releases before that) TypeScript emits readonly T[] for readonly array types. This breaks consumers of declaration files if they use an older version of TypeScript.

This transformer ensures that your declaration files can be used in older TypeScript versions as well.

It does not convert readonly tuple types in the form of readonly [string, number] as this is not supported in older versions of TypeScript.

I wrote this transformer for use with ttypescript.

You can configure it in your tsconfig.json:

{
  "compilerOptions": {
    "declaration": true,
    "plugins": [
      { "transform": "ts-transform-readonly-array", "afterDeclarations": true },
    ]
  }
}

Note that you can use any "type" for the transformer: the default is "type": "program", but it also works with "type": "raw" for example.

Afterwards you run ttsc as you would run tsc.

This package exports the necessary factory function to create the transformer. You can use this function to plug this transformer in any major TypeScript compilation pipeline. Please refer to the API documentation of the tool you are using. Alternatively you can use ttypescript in most tools.