Compilation should not remove uninitialized properties from a class

[flyyuan]

Describe the bug

Compilation should not remove uninitialized properties from a class.
It is useful to have properties in the class that are not assigned initial values. For example, mobx will listen based on the value of the attribute in the class, and if the attribute that is not assigned an initial value is removed, mobx will not be able to listen to the corresponding attribute. Besides, many people are used to declaring class properties without giving them initial values, and swc compilation can create bugs that developers don't expect.

input.js

class IncStore {
  num = 0
  unInitValue
  inc() {
    this.num += 1
    this.unInitValue += 1
  }
}
const incStore = new IncStore()
window.aaa = incStore
export default incStore

output.js (The unInitValue was removed after compilation, expecting it to be there)

function _classCallCheck(instance, Constructor) {
    if (!(instance instanceof Constructor)) {
        throw new TypeError("Cannot call a class as a function");
    }
}
function _defineProperties(target, props) {
    for(var i = 0; i < props.length; i++){
        var descriptor = props[i];
        descriptor.enumerable = descriptor.enumerable || false;
        descriptor.configurable = true;
        if ("value" in descriptor) descriptor.writable = true;
        Object.defineProperty(target, descriptor.key, descriptor);
    }
}
function _createClass(Constructor, protoProps, staticProps) {
    if (protoProps) _defineProperties(Constructor.prototype, protoProps);
    if (staticProps) _defineProperties(Constructor, staticProps);
    return Constructor;
}
var IncStore = /*#__PURE__*/ function() {
    "use strict";
    function IncStore1() {
        _classCallCheck(this, IncStore1);
        this.num = 0;
    }
    _createClass(IncStore1, [
        {
            key: "inc",
            value: function inc() {
                this.num += 1;
                this.unInitValue += 1;
            }
        }
    ]);
    return IncStore1;
}();
var incStore = new IncStore();
window.aaa = incStore;
export { incStore as default };

Input code

class IncStore {
  num = 0
  unInitValue
  inc() {
    this.num += 1
    this.unInitValue += 1
  }
}
const incStore = new IncStore()
window.aaa = incStore
export default incStore

Config

const { config } = require("@swc/core/spack");

module.exports = config({
  //入口文件
  entry: __dirname + "/input.js",
  //输出
  output: {
    path: __dirname + "/dist",
  },
  // swc编译配置
  options:{
    jsc: {
        //解析配置
        parser: {
          syntax: "typescript", //输入文件格式
          tsx: false,    // 是否支持tsx
          dynamicImport: false, //是否支持动态导入
          decorators: false, //是否支持装饰器
        },
        transform: null,
        target: "es5", //转译目标
        loose: false, 
        externalHelpers: false,
        keepClassNames: false
      },
      // 输出文件配置
      module: {
          type: "commonjs",  
          strict: false,
          strictMode: false,
          lazy: false,
          noInterop: false,
          ignoreDynamic: false
        }
  }
});

Playground link

https://github.com/flyyuan/swcUsingDemo

Expected behavior

Expect to compile to include attributes in the class that are not given initial values

Actual behavior

Remove attributes in the class that are not given initial values after compilation

Version

1.2.128

Additional context

No response

[doerme]

m

[erikian]

I've filed an issue on Next.js the other day about this problem (vercel/next.js#32356), I'm pretty sure it affects many other projects as well. Looking forward to this fix.

[kdy1]

I checked this using babel repl and babel emits nothing for uninitialized properties.

[RiESAEX]

tsc doesn't emit it, swc and babel follow its behavior.
microsoft/TypeScript#7777
if you parser.syntax == ecmascript swc won't remove it.
For this case, should we have a lint rule or leave it to the typechecker？

[flyyuan]

I checked this using babel repl and babel emits nothing for uninitialized properties.

Babel do emit uninitialized properties.
I have verified

[ckken]

+1 & mark

[linhaobin]

+1

[Austaras]

Babel won't emit uninitialized field when typescript plugin is enabled. This issue should be closed.

[kdy1]

@Austaras Thank you!

[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.