swc: Class transformer not working with swc

@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!

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 https://github.com/swc-project/swc/issues/2127 in https://github.com/swc-project/swc/pull/3105 was problematic.

I decided to postpone this.

@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 https://github.com/swc-project/swc/issues/2117#issuecomment-906681274

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

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 🆙

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.

Issue on class-transformer side https://github.com/typestack/class-transformer/issues/796 I suppose invalid, and it should be fixed in swc.

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

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.

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. https://github.com/nestjs/nest-cli/issues/731#issuecomment-993936178