confusion about setState on page 411 of the book

[pennsong]

in the api doc: https://reactjs.org/docs/react-component.html#setstate

state is a reference to the component state at the time the change is being applied. It should not be directly mutated. Instead, changes should be represented by building a new object based on the input from state and props. For instance, suppose we wanted to increment a value in state by props.step:
this.setState((state, props) => {
return {counter: state.counter + props.step};
});

in the book "pro-react-16" page 411:

toggleProMode = () => {
this.setState(state => state.proContextData.proMode
= !state.proContextData.proMode);
}

according to the api doc, page 411 is not workable, because in the function

state => state.proContextData.proMode = !state.proContextData.proMode

there is no explicit return, so the return value is the result of 'state.proContextData.proMode = !state.proContextData.proMode', which is !state.proContextData.proMode, let's assume it's 'true', obviously 'true' is not the state object
but I tested the code, it works, could you please explain why? thanks!

[douglasgitpl]

can anyone answer this? I am having the same problem with:
toggleWrap = () => {
this.setState(state => state.wrapContent = !state.wrapContent)
in Chapter 16 page 476

[Chris-MD]

My understanding is this:

If we start with state.proContextData.proMode is true, then = !state.proContextData.proMode prepares the value
{state.proContextData.proMode : false} , but it is returned to state.setState() which is the only allowed method of changing the state properties outside of the constructor.

In this.setState(state => state.proContextData.proMode = !state.proContextData.proMode);
this could be re-expressed as
this.setState(stateObject => state.proContextData.proMode = !stateObject.proContextData.proMode);
where stateObject is the interpolated parameter that you have declared for the lamda notation, however, the assigned to part has to match the state = state as in the constructor initialization of
this.state = {.... proContextData ...}

I find the fact that you can see things like state = state where state are not necessarily the same thing/object as one of the trickiest parts of understanding this declaritive style language.