Allow arithmetic operations on objects that have valueOf() method

[dawidgarus]

Suggestion

🔍 Search Terms

valueOf

✅ 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

Allow arithmetic operations on objects that have valueOf() method which returns type compatible with arithmetic operations

📃 Motivating Example

class NumberWrapper {
    constructor(private val: number) {}
    valueOf(): number {
        return this.val;
    }
}

const a = new NumberWrapper(4);
const b = new NumberWrapper(5);
console.log(a - b);

Currently this produces errors:

    The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.
    The right-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.

But this works perfectly fine with vanilla JavaScript

💻 Use Cases

Creating wrappers for primitive types that could be used as a target for Proxy

[RyanCavanaugh]

Duplicate #2361