Interesting, will figure out a bit later, have a lot on my plate right now.

I'm not sure about this one, like is it something really useful?
I would like to know who else gonna need it. I'm mainly using react with functional programming paradigms (I'm not using mutation at all) so it's hard to tell if that would be something useful. At least I never had a single use case to use something like that.

I once used it in an Angular app.
I use the container/component pattern and mostly work with immutable data. Notably, data coming from the API is marked readonly (and objects coming from Apollo should not be modified since they come from its internal Redux cache).

But in some presentational document I had to mutate an object for two-way binding purposes. So I derived the original type with -readonly, and the presentational component was copying the received readonly-marked object with { ...obj } to create a temporary, safely mutable copy.

Since then I had refactored that part and it no longer exists. That was the only occurrence. So yes it's pretty rare.

const copy = Object.assign({}, this.state); // copy the state of react
copy.options = []; // raise error "TS2540: Cannot assign to 'options' because it is a read-only property"
this.setState({...copy})

It is fixed as follows:

const copy: Mutable<IState> = Object.assign({}, this.state); // copy the state of react
copy.options = []; // not raise error here now
this.setState({...copy})

or

const copy = Object.assign<{}, IState>({}, this.state);

There may be other scenarios but yes, it are not usual cases.

@emilgpa this can be fixed by just const copy = { ...this.state };

@levenleven This is not about whether there is another way to clone an object but that there may be a scenario where you need to be able to mutate an object.
It may be that the person wants to use Object.assign for compatibility reasons (without wanting to use babel or any other tool).

Ok let's include this type, I agree with the @emilgpa argument that there might be some cases where you would want to reverse operation of DeepReadonly helper or clone immutable object to mutate it.

Accepting PRs.

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Would be lovely to have DeepMutable as well 🙂 To mirror DeepReadonly