setInterval in a React app

I see 4 issues with your code:

In your timer method you are always setting your current count to 10
You try to update the state in render method
You do not use setState method to actually change the state
You are not storing your intervalId in the state

Let's try to fix that:

componentDidMount: function() {
   var intervalId = setInterval(this.timer, 1000);
   // store intervalId in the state so it can be accessed later:
   this.setState({intervalId: intervalId});
},

componentWillUnmount: function() {
   // use intervalId from the state to clear the interval
   clearInterval(this.state.intervalId);
},

timer: function() {
   // setState method is used to update the state
   this.setState({ currentCount: this.state.currentCount -1 });
},

render: function() {
    // You do not need to decrease the value here
    return (
      <section>
       {this.state.currentCount}
      </section>
    );
}

This would result in a timer that decreases from 10 to -N. If you want timer that decreases to 0, you can use slightly modified version:

timer: function() {
   var newCount = this.state.currentCount - 1;
   if(newCount >= 0) { 
       this.setState({ currentCount: newCount });
   } else {
       clearInterval(this.state.intervalId);
   }
},

Updated 10-second countdown using class Clock extends Component

import React, { Component } from 'react';

class Clock extends Component {
  constructor(props){
    super(props);
    this.state = {currentCount: 10}
  }
  timer() {
    this.setState({
      currentCount: this.state.currentCount - 1
    })
    if(this.state.currentCount < 1) { 
      clearInterval(this.intervalId);
    }
  }
  componentDidMount() {
    this.intervalId = setInterval(this.timer.bind(this), 1000);
  }
  componentWillUnmount(){
    clearInterval(this.intervalId);
  }
  render() {
    return(
      <div>{this.state.currentCount}</div>
    );
  }
}

module.exports = Clock;

Updated 10-second countdown using Hooks (a new feature proposal that lets you use state and other React features without writing a class. They’re currently in React v16.7.0-alpha).

import React, { useState, useEffect } from 'react';
import ReactDOM from 'react-dom';

const Clock = () => {
    const [currentCount, setCount] = useState(10);
    const timer = () => setCount(currentCount - 1);

    useEffect(
        () => {
            if (currentCount <= 0) {
                return;
            }
            const id = setInterval(timer, 1000);
            return () => clearInterval(id);
        },
        [currentCount]
    );

    return <div>{currentCount}</div>;
};

const App = () => <Clock />;

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

setInterval in a React app

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