Cannot assign to read only property 'closed' of object '[object Object]'

You have to use Object.assign method to change the value of the object as follows,

 Object.assign(target, source);

I believe this happens because you cannot modify an object that is stored in the state (ngRx), you could try something like this instead:

sendProposal() {
  this.store.dispatch(new CloseProposal(Object.assign({}, this.model, {
    details: { closed: true }
  })));
}