Readonly everything by default

Suggestion

🔍 Search Terms

• Readonly by default
• Record readonly by default
• Array readonly by default #32467
• Tuple readonly by default #40316
• Immutable-By-Default Flags #32758

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Right now default is that all record/array/tuple properties are mutable, and if you want any of them to be readonly/immutable you should add readonly flag or use Readonly<...>. My suggestion is add a flag (or something like that) which will "flip" this - it will turn on "assume everything is read only" in TS project(or module) and add a keyword mutable when you want to mark something as mutable.

📃 Motivating Example

When using are not mutating data that much and most of the types are assumed to be immutable while very little is mutable you might accidentally mutate something or when trying to understand portion of code, which mostly uses immutable values but some are mutable, you have one option to use readonly/Readonly.. but that code becomes quite noisy. With this flag you can turn on "readonlyByDefault" flag and everthing will be assumed to be readonly and you could mark mutable fields/values with mutable keyword. This way you would know exactly what's mutable easily and not mutate stuff accidentally.

💻 Use Cases

Probably 99% of react-redux projects do not mutate objects/state or use libraries for immutable structures. Also some teams where folks are using immutable values (and other functional programing practices) would benefit a lot.

I proposed this initially here and then I noticed it had 25 👍 and suggestion to open separate proposal , which I did here.

No files, tests, or compiler entry points are named. Start by reading the proposal and its linked issues, #32467, #40316, and #32758, to understand the related design discussions. Done would require a decided design for project- or module-level readonly defaults and the corresponding mutable escape hatch.