Simple Timer

timer.js

class Component extends React.Component {
  constructor(props) {
    super(props)
    
    this.state = {
      time: null
    }
  }
  
  componentDidMount() {
    const time = this.props.exam.time * 60  // time in minutes * 60 seconds/minute = total seconds
    this.setState({time})
  }
  
  initTimer = () => {
    this.timer = setInterval(() => {
      const {time} = this.state
      if (time === 0) {
        clearInterval(this.timer)
        // end the exam
      }
      this.setState({ time: time - 1 })     // decrease time by one second every second
    }, 1000)
  }
  
  pauseTimer = () => {
    clearInterval(this.timer)
    // show some ui to indicate timer is paused
  }
  render() {
    return (...)
  }
}