Incrementing state value by one using React

Question

In React I am trying to make a button increment a value stored in state. However using the code below function my value is set undefined or NaN when using handleClick.

class QuestionList extends React.Component {
  constructor(props) {
    super(props);
    this.state = {value: 0};

    // This binding is necessary to make `this` work in the callback
    this.handleClick = this.handleClick.bind(this);
  }

   handleClick = (prevState) => {
    this.setState({value: prevState.value + 1});
    console.log(this.state.value)
  }

Can you tell me why this is happening? it should be correct according to the docs here: https://facebook.github.io/react/docs/state-and-lifecycle.html

Answer 1

Because you are using the handleClick function incorrectly. Here:

handleClick = (prevState) => { .... }

prevState will be an event object passed to handleClick function, you need to use prevState with setState, like this:

handleClick = () => {
    this.setState(prevState => {
       return {count: prevState.count + 1}
    })
}

Another issue is, setState is async so console.log(this.state.value) will not print the updated state value, you need to use callback function with setState.

Check more details about async behaviour of setState and how to check updated value.

Check the working solution:

class App extends React.Component {
 
   constructor(props){
       super(props);
       this.state={ count: 1}
   }
 
  onclick(type){
      this.setState(prevState => {
         return {count: type == 'add' ? prevState.count + 1: prevState.count - 1}
      });
  }

   render() {
    return (
      <div>
        Count: {this.state.count}
        <br/>
        <div style={{marginTop: '100px'}}/>
        <input type='button' onClick={this.onclick.bind(this, 'add')} value='Inc'/>
        <input type='button' onClick={this.onclick.bind(this, 'sub')} value='Dec'/>
       </div>
     )
   }
}

ReactDOM.render(
  <App />,
  document.getElementById('container')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

<div id='container'></div>