PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

SWC adopts ESNext behaviour of explicitly setting fields as undefined by default. Disabling this behaviour aligns better with TS that targets ES2021.

The result is as following:

import { plainToInstance } from "class-transformer";

class SomeClass {
  public aNumber: number;
  public aString: string;
}

const partialA = plainToInstance(SomeClass, { aNumber: 1 });
const partialB = plainToInstance(SomeClass, { aString: "abc" });
console.log({ ...partialA, ...partialB });
// Expected: { aNumber: 1, aString: 'abc' }
// Actual:   { aNumber: undefined, aString: 'abc' }

Issue Number: N/A

What is the new behavior?

console.log({ ...partialA, ...partialB });
// Logs: { aNumber: 1, aString: 'abc' }

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Relates to swc-project/swc#3459

Reproduction repo available https://github.com/tuxmachine/swc-class-transformer