Repository: TypeScript
Issue: C++-style `const` modifier on class members
Labels: Suggestion,Awaiting More Feedback

### 🔍 Search Terms

const method parameter keyword readonly class
C++ like const methods
class method with const keyword
"as const" for class type


### ✅ Viability Checklist

- [X] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [X] This wouldn't change the runtime behavior of existing JavaScript code
- [X] This could be implemented without emitting different JS based on the types of the expressions
- [X] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [X] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [X] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

~~I'm sure I've seen this suggestion somewhere before but I couldn't find it~~ https://github.com/microsoft/TypeScript/issues/35313

Allow using `as const` on class objects by annotating methods as `const` (syntax tbd).

A const method would (like in C++) not allow modifying properties of `this` or calling any method not also marked as `const`.

An object marked with `as const` would similarly be readonly and only methods marked as `const` would be callable.

### 📃 Motivating Example

Keeping track of mutable vs immutable can be tricky when you're not used to the library you're using (or if its poorly written). It's very important to get it right in many libraries (`react` state, `vue` refs (shallow vs deep), `preact/signals`) where modifying values may or may not be required.

Debugging mutability bugs can be a pain sometimes too

```ts
const start = new Point(0, 0);
const line: Point[] = [];

for (let i = 0; i < 10; i++) {
  line.push(start.add(i * 10, Math.random()));
}

// equivalent to
const line = new Array(10).fill(new Point(450, Math.random()));
```

### Declaration syntax 1 using a `const` decoration

```ts
class Point {
	constructor(public x: number, public y: number) { }
	const clone(): Point { ... }

	add(other: PointLike): this { ... }
}

const p = new Point(0, 0) as const; // type is { readonly x: number, readonly y: number, readonly clone(): Point } (`add` has been removed)
```

### Declaration syntax 2 using `this: const` parameter

```ts
class Point {
	constructor(public x: number, public y: number) { }
	clone(this: const): Point { ... }

	add(other: PointLike): this { ... }
}

const p = new Point(0, 0) as const; // type is { readonly x: number, readonly y: number, readonly clone(): Point } (`add` has been removed)
```

I personally much prefer the first syntax where `const` is a decoration.

### Parameter syntax 1

```ts
function noModifyPoint(p: const Point) {
	p.clone().add(1, 1);
}

function modifyPoint(p: Point) {
	p.add(1, 1);
}

const p = new Point(0, 0) as const;

noModifyPoint(p); // ok
modifyPoint(p); // Argument of type `const Point` is not assignable to parameter of type `Point`. Type `const Point` is missing the following properties from type `Point`: add
```

Not to be confused with

```ts
function foo() {
	const p = arguments[0];
	//...
}
```

(as an aside a more appropriate syntax for that would be `foo(const p: Point)` or `foo(const p: const Point)` but that's OoS)

### Parameter syntax 2

```ts
function noModifyPoint(p: Readonly<Point>) {
	p.clone().add(1, 1);
}

function modifyPoint(p: Point) {
	p.add(1, 1);
}

const p = new Point(0, 0) as const;

noModifyPoint(p); // ok
modifyPoint(p); // Argument of type `Readonly<Point>` is not assignable to parameter of type `Point`. Type `Readonly<Point>` is missing the following properties from type `Point`: add
```

Again I prefer syntax 1. The const type inference needs to happen in the compiler, making it look like a utility type would be confusing. The use of `const` also mirrors the declaration syntax 1.

The inference of the new readonly type should :tm: be simple enough, convert any r/w properties to readonly, and remove any functions not marked as const.

For simplicity arrow method properties should be handled as any other property would.

```ts
class Point {
	constructor(public x: number, public y: number) { }
	clone(this: const): Point { ... }

	unsafelyModifyThis: () => { this.x = 10; }
}

const p = new Point(0, 0) as const;

p.unsafelyModifyThis(); // OK
```

The reason I suggest this is only because there needs to be a line drawn somewhere as to how complex this feature becomes. As mentioned in [TypeScripts third non-goal](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals#:~:text=Apply%20a%20sound%20or%20%22provably%20correct%22%20type%20system.%20Instead%2C%20strike%20a%20balance%20between%20correctness%20and%20productivity) this is a tradeoff between usefulness and simplicity.

Its already possible to break the type system and its up to the programmers to not do stupid things.

### Other types

```ts
const a1 = [1, 2, 3] as const;
const a2 = [1, 2, 3] as Readonly<[number, number, number]>;

function aFunc1<T>(p: const Array<T>) {
	// syntactically valid but is probably useful since Array doesn't have any built-in const decorations
	// the resulting type would be `{ readonly [index: number]: T; readonly length: number }`
	// also not sure how it would work with the `ReadonlyArray<T>` built-in type
	...
}

function prim(n: const number) { ... } //error
```

### 💻 Use Cases

1. What do you want to use this for?
	This would be most useful for data classes (aka structs) that represent non-primative data structures.
2. What shortcomings exist with current approaches?
	Workarounds are not DRY, requiring manual definition for const variants of all types
3. What workarounds are you using in the meantime?
	Manually defining a `ReadonlyPoint` variation and casting to it.

	```ts
	interface ReadonlyPoint {
		readonly x: number;
		readonly y: number;
		readonly xy: [number, number];
	
		clone(): Point;
	}
	
	class Point implements ReadonlyPoint {
		constructor(public x: number, public y: number) { }
	
		clone(): Point {
			return new Point(this.x, this.y);
		}
	
		add(other: PointLike): this {
			this.x += other.x;
			this.y += other.y;
			return this;
		}
	
		//...
	}
	
	const p = new Point(0, 0) as ReadonlyPoint;
	
	p.add(p); // error
	```