Recognize property with name defined by constant in Object.defineProperty

[sKopheK]

Search Terms

defineProperty
#28694 is touching same domain

Suggestion

(Note: I'm using TS in JS files, have basically no knowledge about "pure" TS)
Compiler should recognize prototype property added using Object.defineProperty with name defined not only by string constant but also a string variable, e.g.:

const propName = 'myProp';
Object.defineProperty(Test.prototype, propName, { get: () => 'myVal' });

Playground Link
(included also desired behavior for decorators, but as it's experimental feature, you won't propably even consider it)

I understand it's not always possible to make sure variable with property name is not reassigned during runtime, but it seems that TS compiler can handle detection of "static" variables that have known value during compilation.

Use Cases

This way we could define class properties using "single source" and not duplicating code. For example a class providing access to key-value pairs object in a dotted notation instead of string values for keys, whilst the object can be used for another purpose.

Examples

// i can use these somewhere else, e.g. as a type
export const defaults = {
    one: 1,
    two: 2
}

// properties added in defineProperty
export class Test { 
    // i don't want to repeat myself
    one = defaults.one;

    /** @param {keyof defaults} key */
    get(key) {
        return defaults[key];
    }
}
Object.keys(defaults).forEach((key) =>
    Object.defineProperty(Test.prototype, key, { get: () => defaults[key] })
);

const inst = new Test();
inst.one;   // who would not like this?
inst.get('one');    // instead of this

Playground Link

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

[sKopheK]

How much ugly is this workaround :)?

type extraProps = {
    DecoratedProp: number;
    ExtendedProp: string;
}

function decorator(target: Function) {
    target.prototype.DecoratedProp = 2;
}

@decorator
class Test {
    Prop: number;
    constructor() {
        this.Prop = 1;
    }    
}

Object.defineProperty(Test.prototype, 'ExtendedProp', { writable: true });

const DecoratedTest = Test as ((new () => Test & extraProps) & typeof Test);

const inst = new DecoratedTest();
console.log(inst.DecoratedProp);    // outputs "2"
inst.ExtendedProp = 'test';
console.log(inst.ExtendedProp);    // outputs "test"

Playground Link

EDIT: Added a more detailed example with static properties and generics: Playground Link
Is there a way how to make generics work with a single parameter?

JavaScript version: Playground Link (had to remove decorator as it's not working even tho decorator support was enabled)