swc generates a different prototype chain than tsc

[rafaelliu]

Describe the bug

The way swc defines class properties generates a different prototype chain from tsc, with semantic differences.

swc uses Object.defineProperty to define a property (even if not initialized) at the class object, while tsc either doesn't define any property (if it's not initialized) or initializes it with this.

One issue is user-define properties defined elsewhere in the prototype chain are never reached. Also, the initialization of the property for a code compiled with tsc will use any current set accessor (or an error will be throw if none is set).

Input code

class MyClass {
   myProp: number;
}

Object.defineProperty(MyClass.prototype, 'myProp', { value: 42 })
console.log(new MyClass().myProp)

### Config

```json
Using ts-node with `swc: true`

Playground link

No response

Expected behavior

same as tsc, generate a JS not defining any property:

let MyClass = class MyClass {
};
export { MyClass };

Which prints "42"

Actual behavior

swc generates a property, and will use Object.defineProperty:

export let MyClass = class MyClass {
    constructor(){
        _define_property(this, "myProp", void 0);
    }
};

Which prints "undefined"

Version

1.3.56

Additional context

Even when initializing the property, tsc is more flexible in that it uses this:

let MyClass = class MyClass {
    constructor() {
        this.componentManager = 42;
    }
};
export { MyClass };

swc:

export let MyClass = class MyClass {
    constructor(){
        _define_property(this, "myProp", 42);
    }
};

[rafaelliu]

Hi @kdy1, it took a while to troubleshoot the issue, and then some more to report it compiling different cases and comparing outputs.

I understand its your prerogative to choose what goes and what doesn’t go into swc, but would you care provide a word or two?

[kdy1]

@rafaelliu Search before filing an issue.

[TiddoLangerak]

For future reference, as this issue is a dead end at the moment.

The behaviour can be toggled by disabling the useDefineForClassFields option.

[rafaelliu]

Following @TiddoLangerak spirit of collaboration, the problem is ts-node doesn’t allow that to be set. There’s a PR open, original issue here: TypeStrong/ts-node#1856

This wasn’t the default for swc and used to work, but it changed on version 1.3.39 which broke things.

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.