setInterval in a React app

I'm still fairly new at React, but I've been grinding along slowly and I've encountered something I'm stuck on.

I am trying to build a "timer" component in React, and to be honest I don't know if I'm doing this right (or efficiently). In my code below, I set the state to return an object { currentCount: 10 } and have been toying with componentDidMount, componentWillUnmount, and render and I can only get the state to "count down" from 10 to 9.

Two-part question: What am I getting wrong? And, is there a more efficient way of going about using setTimeout (rather than using componentDidMount & componentWillUnmount)?

Thank you in advance.

import React from 'react';

var Clock = React.createClass({

  getInitialState: function() {
    return { currentCount: 10 };
  },

  componentDidMount: function() {
    this.countdown = setInterval(this.timer, 1000);
  },

  componentWillUnmount: function() {
    clearInterval(this.countdown);
  },

  timer: function() {
    this.setState({ currentCount: 10 });
  },

  render: function() {
    var displayCount = this.state.currentCount--;
    return (
      <section>
        {displayCount}
      </section>
    );
  }

});

module.exports = Clock;

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'));

If anyone is looking for a React Hook approach to implementing setInterval. Dan Abramov talked about it on his blog. Check it out if you want a good read about the subject including a Class approach. Basically the code is a custom Hook that turns setInterval as declarative.

function useInterval(callback, delay) {
  const savedCallback = useRef();

  // Remember the latest callback.
  useEffect(() => {
    savedCallback.current = callback;
  }, [callback]);

  // Set up the interval.
  useEffect(() => {
    function tick() {
      savedCallback.current();
    }
    if (delay !== null) {
      let id = setInterval(tick, delay);
      return () => clearInterval(id);
    }
  }, [delay]);
}

Also posting the CodeSandbox link for convenience: https://codesandbox.io/s/105x531vkq

Manage setInterval with React Hooks:

  const [seconds, setSeconds] = useState(0)

  const interval = useRef(null)

  useEffect(() => { if (seconds === 60) stopCounter() }, [seconds])

  const startCounter = () => interval.current = setInterval(() => {
    setSeconds(prevState => prevState + 1)
  }, 1000)

  const stopCounter = () => clearInterval(interval.current)

Thanks @dotnetom, @greg-herbowicz

If it returns "this.state is undefined" - bind timer function:

constructor(props){
    super(props);
    this.state = {currentCount: 10}
    this.timer = this.timer.bind(this)
}

The easy thing to do is to add it to the window variable.

useEffect(() => {
    window.interval23 = setInterval(
      () => setState('something'),
      2500
    )
    return () => {
      clearInterval(window.interval23)
    }
 }, [])

but make sure whatever you create with the window variable, keep it as unique as possible because the window variable might interrupt in libraries if that variable already exists.

Updating state every second in the react class. Note the my index.js passes a function that return current time.

import React from "react";

class App extends React.Component {
  constructor(props){
    super(props)
    
    this.state = {
      time: this.props.time,
      
    }        
  }
  updateMe() {
    setInterval(()=>{this.setState({time:this.state.time})},1000)        
  }
  render(){
  return (
    <div className="container">
      <h1>{this.state.time()}</h1>
      <button onClick={() => this.updateMe()}>Get Time</button>
    </div>
  );
}
}
export default App;

If you are using Dan Abramov useInterval hook and want to manually cancel the current interval you just need to call the hook again passing null as delay variable.

You can check a working example here https://codesandbox.io/s/useinterval-cancel-interval-dan-abramov-extended-oe45z?file=/src/index.js

You can use interval to set state by creating what I call a fake recursion by combining setTimeout and useEffect

  import {useEffect,useState} from 'react'

  const [state,setState]=useState(0)

  function Interval(){
      setTimeout(()=>{
         setState(state+1)
       },2000)
  }
   useEffect(()=>Interval(),[state])

 //this code runs repeatedly in interval of 2 seconds

import React, { useState, useEffect } from "react";

export const Count = () => {
const [currentCount, setCount] = useState(1);

const timer = () => setCount(currentCount + 1);

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

console.log(currentCount)

return <div>Count : - {currentCount}</div>;
};