extends with OmitType removes methods from Base class

[mohsensaremi]

I'm submitting a...

[ ] Regression 
[x] Bug report
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

When extends using OmitType(), the derived class does not contain Base class methods.

Expected behavior

Expect derived class to have parent class methods

Minimal reproduction of the problem with instructions

import { IsString } from 'class-validator';
import { OmitType } from '@nestjs/mapped-types';
import { plainToClass } from 'class-transformer';

class BaseClass {
  @IsString()
  a: string;

  @IsString()
  b: string;

  someMethod() {
    return 'this is a method';
  }
}

class DerivedClass extends OmitType(BaseClass, ['a']) {}

const instance = plainToClass(DerivedClass, {
  b: 'some value',
});

console.log(instance.someMethod());

expect this code to log this is a method but it will throw the following error:

console.log(instance.someMethod());
                     ^
TypeError: instance.someMethod is not a function
...

What is the motivation / use case for changing the behavior?

It should work as normal extends.

Environment

Nest version: ^8.0.0
@nestjs/mapped-types:  ^1.0.0

 
For Tooling issues:
- Node version: v14.16.1
- Platform:  Mac

Others:

[micalevisk]

This is expected since methods are just properties.

Thus, you must do something like OmitType(BaseClass, ['a', 'someMethod'])

[mohsensaremi]

@micalevisk
I want to someMethod inherit from BaseClass but you Omit it!
Using OmitType(BaseClass, ['a', 'someMethod']) did not help.

[kamilmysliwiec]

Please search through some of our old issues on this (this has been discussed several times in the past).