Uninitialized class property is defined with value "void 0" in es2015

[smvv]

Describe the bug

When transpiling to es2015, swc initializes an uninitialized class property to undefined:

// [...]
class Base {
    constructor(){
        _define_property(this, "data", 1);
    }
}
export class Foo extends Base {
    constructor(...args){
        super(...args);
        _define_property(this, "data", void 0);
    }
}

When es2022 is used, it does not initialize the property:

class Base {
    data = 1;
}
export class Foo extends Base {
    data;
}

Similarly, tsc and esbuild skip the initialization.

Input code

class Base {
  data: number = 1
}

export class Foo extends Base {
    data!: number
}

Config

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false
    },
    "target": "es2015",
    "loose": false,
    "minify": {
      "compress": false,
      "mangle": false
    }
  },
  "module": {
    "type": "es6"
  },
  "minify": false,
  "isModule": true
}

Playground link

https://play.swc.rs/?version=1.3.87&code=H4sIAAAAAAAAA0vOSSwuVnBKLE5VqOZSUEhJLEm0UsgrzU1KLVKwVTDkquXiSq0oyC8qUUgGq3TLz1dIrShJzUtB0gXRpwjTyFULABjS7aVWAAAA&config=H4sIAAAAAAAAA1VPSQ7CMAy85xWRzxwACQ78gUdEwa2Csil2JaKqfydpk0Jv9iye8SykhDdpeMi5jGWJKhGmfS8IZc%2FqUxDgHJF0MpHh1FmmSg3KEq7QsjHAKo3I1YV0PV9uzQE2BMLuaJgz3gz5P1MHFxMSHYVVqvxo8ZgoWiq48JpWsv1S%2B24N7vAT9bD9MBh6dienCcXyBbnuEC8XAQAA

SWC Info output

No response

Expected behavior

Skip the initialization of uninitialized class properties when target is es2015-2021 output.

It's not initializing to undefined when target is es2022.

Actual behavior

By setting the uninitialized property's value to undefined, it overrides the property value set in the base class.

Version

1.3.87

Additional context

Related issue: #3232

[kdy1]

#7435

[magic-akari]

You can run this code in modern browsers with native class support.
You will get undefined.

class Base {
    data = 1;
}
class Foo extends Base {
    data;
}

console.log(new Foo().data); // undefined

TypeScript support class field syntax before it is standardized.
And TypeScript's class field has different semantics with native class field.

This is why useDefineForClassFields exists.
When target is set to es2022 or higher, TypeScript will use native class field semantics.

But you should not rely on the behavior of target default value.
You should always set useDefineForClassFields to true or false explicitly.
Once it works in TypeScript, it will work in swc.

[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.