no-access-state-in-setstate.md

Prevent using this.state within a this.setState (react/no-access-state-in-setstate)

This rule should prevent usage of this.state inside setState calls. Such usage of this.state might result in errors when two state calls are called in batch and thus referencing old state and not the current state. An example can be an increment function:

function increment() {
  this.setState({value: this.state.value + 1});
}

If two setState operations are grouped together in a batch, they both evaluate the old state. Given that state.value is 1:

this.setState({value: this.state.value + 1}) // 2
this.setState({value: this.state.value + 1}) // 2, not 3

This can be avoided with using callbacks which takes the previous state as first argument:

function increment() {
  this.setState(prevState => ({value: prevState.value + 1}));
}

Then react will call the argument with the correct and updated state, even when things happen in batches. And the example above will be something like:

setState({value: 1 + 1})
setState({value: 2 + 1})