Suggestion

🔍 Search Terms

• readonly method
• readonly this
• sealed class
• sealed this
• readonly reference Add pure and immutable keywords to ensure code has no unintended side-effects #17181 (this issue is simmilar but more restrictive)

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code.
  • It can or cannot be. It depends on wich name we choose
• 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

Have a way to check readonly object references (including parameters, and this parameter). That allows to check than a function does not modifys an object.

📃 Motivating Example

class Figure{
    constructor (public name: string){}
    checkName(){
        this.name += ' !'
    }
    readonlyMethod logAccess(){
        console.log('one instance of Figure was accesed');
    }
    readonlyMethod safeCheck(){
       // @ts-expect-error
        this.name += ' x'  // this is readonly in a readonlyMethod context
    }
}

function showRO(figure: readonlyReference Figure){
    console.log(figure.name);
    // @ts-expect-error
    figure.name+=" x";  // if figure is a readonly reference you can use it to modify the object
    // @ts-expect-error
    figure.checkName();  // if figure is a readonly reference you can use it to modify the object calling a method
    figure.logAccess();  // ok logAccess is a readonly method
}

var x = new Figure('triangle');

showRO(x);

var z: readonlyReference Figure = x; // ok, you can assign a writeable object to a readonlyReference

// @ts-expect-error
var y: Figure = z ;  // you cannot do the oposite. 

💻 Use Cases

Actually I'am ussing Readonly aproach, but it not contemplates all cases.

play example

I am trying with Object.seal and Proxyies .