There could be an option to emit a validation error if the object contains a property it shouldn't (e.g. the property isn't declared and decorated in the class).
Maybe it should be a part of your https://github.com/pleerock/class-transformer library instead of this one, I'm not sure.
decide where it should be and create more detailed proposal
This is now possible with the whitelist option.
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decide where it should be and create more detailed proposal