What is prevState in ReactJS?

prevState is a name that you have given to the argument passed to setState callback function. What it holds is the value of state before the setState was triggered by React; Since setState does batching, its sometimes important to know what the previous state was when you want to update the new state based on the previous state value.

So if multiple setState calls are updating the same state, batching setState calls may lead to incorrect state being set. Consider an example:

state = {
   count: 0
}
updateCount = () => {
    this.setState({ count: this.state.count + 1});
    this.setState({ count: this.state.count + 1});
    this.setState({ count: this.state.count + 1});
    this.setState({ count: this.state.count + 1});
}

In the above code you might expect the value of count to be 4 but it would actually be 1 since the last call to setState will override any previous value during batching. A way to solve this to use functional setState:

updateCount = () => {
    this.setState(prevstate => ({ count: prevstate.count + 1}));
    this.setState(prevstate => ({ count: prevstate.count + 1}));
    this.setState(prevstate => ({ count: prevstate.count + 1}));
    this.setState(prevstate => ({ count: prevstate.count + 1}));
}

You use it when you want to override the current state with the last state's parameters.

From React docs :

According to the React docs "React may batch multiple setState() calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state."

"To fix it, use a second form of setState() that accepts a function rather than an object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument"

Link

Here is a demo with a commented-out code to give you more information: http://codepen.io/PiotrBerebecki/pen/rrGWjm

constructor() {
  super();
  this.state = {
    value: 0
  }
}

React docs: https://facebook.github.io/react/docs/reusable-components.html#es6-classes

The [React ES6 class] API is similar to React.createClass with the exception of getInitialState. Instead of providing a separate getInitialState method, you set up your own state property in the constructor.

Where does prevState come from?

The prevState comes from the setState api: https://facebook.github.io/react/docs/component-api.html#setstate

It's also possible to pass a function with the signature function(state, props). This can be useful in some cases when you want to enqueue an atomic update that consults the previous value of state+props before setting any values. For instance, suppose we wanted to increment a value in state:

this.setState(function(previousState, currentProps) {
  return {
     value: previousState.value + 1
  };
});