Subclass field declaration

main.ts

import { Expose, Type, plainToClass } from 'class-transformer';
import { Equals, IsString } from 'class-validator';
import 'reflect-metadata';

abstract class FooBar {
    @IsString()
    @Expose()
    type: string;
}

class Foo extends FooBar {
    @Equals('foo')
    @Expose()
    type: 'foo';
}

class Bar extends FooBar {
    @Equals('bar')
    @Expose()
    type: 'bar';
}

class Item {
    @Type(() => FooBar, {
        discriminator: {
            property: 'type',
            subTypes: [
                { name: 'foo', value: Foo },
                { name: 'bar', value: Bar },
            ],
        },
        keepDiscriminatorProperty: true,
    })
    @Expose()
    fooBar: FooBar;
}

const items: Item[] = [
    plainToClass(Item, { fooBar: { type: 'foo' } }),
    plainToClass(Item, { fooBar: { type: 'bar' } }),
];

console.log(items);

package.json

{
  "dependencies": {
    "class-transformer": "^0.5.1",
    "class-validator": "^0.14.0",
    "reflect-metadata": "^0.1.13"
  },
  "devDependencies": {
    "typescript": "^5.2.2"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "lib": ["dom", "esnext"],
    "module": "commonjs",
    "outDir": "dist",
    "target": "es2022"
  },
  "include": ["main.ts"]
}