Class transformer not working with swc

[DanielRamosAcosta]

Describe the bug

When applying the plainToClass function to a class defined with class-validator, it returns the class with no attributes

Input code

Here is a repo with a test reproducing the error

const usersOptionsInput = plainToClass(GetUsersOptionsInput, {
  query: "Query",
  pagination: { skip: 0, limit: 20 },
});

expect(usersOptionsInput.query).toEqual("Query"); // error

Config

You can also find the config at the repo reproducing the error

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false,
      "decorators": true
    },
    "transform": {
      "legacyDecorator": true,
      "decoratorMetadata": true
    },
    "target": "es2018"
  },
  "module": {
    "type": "commonjs",
    "noInterop": true
  }
}

Expected behavior
I would expect the instance of the class to have all the defined attributes.

Version
The version of @swc/core: 1.2.80

Additional context

Just one more time, I created this repo in order to reproduce the error

This error is related to #1362

[tonivj5]

I'm coming from #1362 and this doesn't work #1362 (comment)

Good repro btw @DanielRamosAcosta!

[tonivj5]

I've been checking the generated code, and there's a problematic part.

Minimal repro

• Source

import { IsOptional, IsString } from 'class-validator';

export class GetUsersOptionsInput {
  @IsString()
  @IsOptional()
  public query?: string;
  // it works
  // public query?: string = undefined;
}

• Generated

"use strict";
Object.defineProperty(exports, "__esModule", {
    value: true
});
exports.GetUsersOptionsInput = void 0;
var _classValidator = require("class-validator");
function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) {
    var desc = {
    };
    Object.keys(descriptor).forEach(function(key) {
        desc[key] = descriptor[key];
    });
    desc.enumerable = !!desc.enumerable;
    desc.configurable = !!desc.configurable;
    if ("value" in desc || desc.initializer) {
        desc.writable = true;
    }
    desc = decorators.slice().reverse().reduce(function(desc, decorator) {
        return decorator ? decorator(target, property, desc) || desc : desc;
    }, desc);
    if (context && desc.initializer !== void 0) {
        desc.value = desc.initializer ? desc.initializer.call(context) : void 0;
        desc.initializer = undefined;
    }
    // IF NO INITILIZAER, THE RETURNED DESCRIPTOR IS NULL
    if (desc.initializer === void 0) {
        Object.defineProperty(target, property, desc);
        desc = null;
    }
    return desc;
}
function _initializerDefineProperty(target, property, descriptor, context) {
    if (!descriptor) return;
    Object.defineProperty(target, property, {
        enumerable: descriptor.enumerable,
        configurable: descriptor.configurable,
        writable: descriptor.writable,
        value: descriptor.initializer ? descriptor.initializer.call(context) : void 0
    });
}
var _class, _descriptor, _dec, _dec1, _dec2;
let GetUsersOptionsInput = ((_class = class GetUsersOptionsInput1 {
    constructor(){
        _initializerDefineProperty(this, "query", _descriptor/*THIS DESCRIPTOR IS NULL*/, this);
    }
}) || _class, _dec = (0, _classValidator).IsString(), _dec1 = (0, _classValidator).IsOptional(), _dec2 = typeof Reflect !== "undefined" && typeof Reflect.metadata === "function" && Reflect.metadata("design:type", String), _descriptor = _applyDecoratedDescriptor(_class.prototype, "query", [
    _dec,
    _dec1,
    _dec2
], {
    configurable: true,
    enumerable: true,
    writable: true,
    initializer: void 0 // THE INITIALIZER IS UNDEFINED
}), _class);
exports.GetUsersOptionsInput = GetUsersOptionsInput;

This is the problematic part from _applyDecoratedDescriptor, this branch is executed by initializer: void 0

    if (desc.initializer === void 0) {
        Object.defineProperty(target, property, desc);
        desc = null;
    }

When the property decorated has not initializer the descriptor decorator returned is null. If we remove that if, the repro code works and tests pass.

I don't know exactly why that check is neccesary, but there is the problem.

I hope it helps @kdy1! Excellent project btw 🆙

[DanielRamosAcosta]

Great work @tonivj5! i tried diving into the generated code but I didn't reach any conclusions, thanks for your insight!

[unlight]

Issue on class-transformer side typestack/class-transformer#796 I suppose invalid, and it should be fixed in swc.

[tonivj5]

@unlight if class-validator works perfect with tsc and fail with swc. I think it's a problem of swc handling decorators...

[kdy1]

@tonivj5 Did you try importing reflect-metadata?

[tonivj5]

@kdy1 yep! It doesn't work.

This code does not work:

import "reflect-metadata";
import { IsOptional, IsString } from "class-validator";

export class GetUsersOptionsInput {
  @IsString()
  @IsOptional()
  public query?: string;
}

This code does work:

import "reflect-metadata";
import { IsOptional, IsString } from "class-validator";

export class GetUsersOptionsInput {
  @IsString()
  @IsOptional()
  public query?: string = undefined;
}

I did a deeper research here #2117 (comment)

[immasandwich]

@tonivj5 Thank you so much for that workaround!

@kdy1 Fixing this would make a world of difference for anyone part of the Nest.js community, it'd be hugely appreciated!

[charlescapps]

+1, it would be great to fix this. We use class-validator pretty heavily, and we would like to run our unit tests with ts-node + swc since it's dramatically faster than using the default transpiler with ts-node.

[RIP21]

If you're struggling with this issue here is the way to fix it so it's almost 0 pain while we're waiting for fix.
nestjs/nest-cli#731 (comment)

[acoroleu-tempus]

this is blocking for us to migrate to swc, company wide

[RIP21]

@acoroleu-tempus use the workaround I mentioned above.

[kdy1]

I investigated this.

For the initializer property of the property descriptor for fields without an initializer, babel uses null while swc emits void 0. _applyDecoratedDescriptor returns null if initializer is undefined, so the property descriptor used for _initializerDefineProperty while instantiating a class is an object for babel while being null for swc.

https://github.com/babel/babel/blob/69ae3b5a250d60618702751aa2aa4f56ccd43988/packages/babel-plugin-proposal-decorators/src/transformer-legacy.ts#L146-L152

I tried changing the value to null, but it broke other tests. Namely, the tests I added for #2127 in #3105 was problematic.

I decided to postpone this.

[minardimedia]

There is also a similar issue when upgrading to Next js 12, they use SWC and support legacy decorators but the @expose() and @exclude() decorators also seem to be falling. they return undefined attributes.

[Geebrox]

Coming from: https://stackoverflow.com/questions/70956406/plaintoclass-from-class-transform-converts-incorrecly
Any progress or possible workaround on this issue? Tried to use esbuild and plainToClass method works correctly.

[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.