Codefix: convert `Type[]` to `readonly Type[]`

Search Terms

codefix readonlyarray readonly array immutable

Suggestion

It'd be nice to have a codefix for converting non-readonly arrays to readonly.

Use Cases

When converting/refactoring code to be more immutable/readonly-friendly, it's a bit of a pain to manually add these annotations.

Examples

Given this:

function takesArrays(items: /*| |*/string[], moreItems: /*| |*/string[]) { /* ... */ }

...having a codefix to make one or all parameters readonly would be useful:

function takesArrays(items: readonly string[], moreItems: readonly string[]) { /* ... */ }

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 from the issue's TypeScript examples and the existing codefix infrastructure for related type annotations. Identify the codefix entry point and its tests, then implement conversion for one or all applicable parameters and verify that the resulting types use readonly array syntax.