Class component

class-component.js

import React, { Component } from "react";

class WindowSize extends Component {
  state = {
    width: window.innerWidth,
    height: window.innerHeight
  };

  handleResize = () => this.setState({ 
      width: window.innerWidth, 
      height: window.innerHeight
  });

  componentDidMount() {
    window.addEventListener("resize", this.handleResize);
  }

  componentWillUnmount() {
    window.removeEventListener("resize", this.handleResize);
  }

  render() {
    const { width, height } = this.state;
    return (
      <>
        <h2>Window size: {width} x {height}</h2>
      </>
    );
  }
}

export default WindowSize;