Stopwatch full updater

full-update-function.js

class StopWatch extends React.Component {
  state = { lapse: 0, running: false };
  handleRunClick = () => {
    //updater function
    this.setState(state => {
      if (state.running) {
        clearInterval(this.timer);
      } else {
        const startTime = Date.now() - this.state.lapse;
        this.timer = setInterval(() => {
          this.setState({
            lapse: Date.now() - startTime
          });
        });
      }
      // returning a new state to not mutate our original state
      return { running: !state.running };
    });
  };