Experiment: Making TypeScript Immutable-by-Default

Summary

Experiment: making TypeScript immutable-by-defaultI like programming languages where variables are immutable by default. For example, in Rust, let declares an immutable variable and let mut declares a mutable one. I’ve long wanted this in other languages, like TypeScript, which is mutable by default—the opposite of what I want!I wondered: is it possible to make TypeScript values immutable by default?My goal was to do this purely with TypeScript, without changing TypeScript itself. That meant no lint rules or other tools. I chose this because I wanted this solution to be as “pure” as possible…and it also sounded more fun.I spent an evening trying to do this. I failed but made progress! I made arrays and Records immutable by default, but I couldn’t get it working for regular objects. If you figure out how to do this completely, please contact me—I must know!Step 1: obliterate the built-in librariesTypeScript has built-in type definitions for JavaScript APIs like Array and Date and String. If you’ve ever changed the target or lib options in your TSConfig, you’ve tweaked which of these definitions are included. For example, you might add the “ES2024” library if you’re targeting a newer runtime.My goal was to swap the built-in libraries with an immutable-by-default replacement.The first step was to stop using any of the built-in libraries. I set the noLib flag in my TSConfig, like this:{ "compilerOptions": { "noLib": true } } Then I wrote a very simple script and put it in test.ts:console.log("Hello world!"); When I ran tsc, it gave a bunch of errors:Cannot find global type 'Array'. Cannot find global type 'Boolean'. Cannot find global type 'Function'. Cannot find global type 'IArguments'. Cannot find global type 'Number'. Cannot find global type 'Object'. Cannot find global type 'RegExp'. Cannot find global type 'String'. Progress! I had successfully obliterated any default TypeScript libraries, which I could tell because it couldn’t find core types like String or ...