How to allow null, but forbid undefined?

e.g. for database rows, we may need nullable properties that must not be undefined:

class DbRow {
  @IsNumber()
  id!: number;

  @IsNumber()
  numNullable!: number | null;
}

So numNullable can be a number or null - but it must never be undefined.

How can we express this in class-validator?

• adding @Optional() does not work, because that would also allow undefined
• I also had no luck with a custom validator

It turns out that this is possible by using conditional validation ValidateIf:

class DbRow {
  @IsNumber()
  id!: number;

  @IsNumber()
  @ValidateIf((object, value) => value !== null)
  numNullable!: number | null;
}

Here is a stackblitz example