App.js

import React, { Component } from "react";
import "./App.css";
import EasyTimer from "easytimer";

class App extends Component {
  constructor() {
    super();

    this.state = { timer: null, timeValues: "" };
  }

  componentDidMount() {
    let timer = new EasyTimer();
    this.setState({ timer: timer });
    timer.start();
    timer.addEventListener("secondsUpdated", this.tick);
  }

  tick() {
    const { timer } = this.state;
    const timeValues = timer.getTimeValues().toString();
    this.setState({ timeValues: timeValues });
    console.log(`timer: ${timer}`);
  }

  render() {
    return <div className="App">{this.state.timeValues}</div>;
  }
}

export default App;