React.js ライブラリ「react-cool-dimensions」を使って要素のサイズを測定する

create-react-app react-app

## 作成したプロジェクトに移動
cd react-app
 
## インストール
npm install react-cool-dimensions

import React, { useRef }  from 'react';
import useDimensions from 'react-cool-dimensions';

const Sample = () => {

  const ref = useRef();
  const { width, height } = useDimensions(ref, {});
  
  return (    
    <div>
      <div style={{
        width: "500px",
        height: "500px",
        padding: "30px",
        border: "5px solid grey",
      }}
      ref={ref}>
        Hi! My width is {width}px and height is {height}px
      </div>
    </div>
  );
}

export default Sample;

import React from 'react';
import Sample from './sample';
import './App.css';

function App() {
  const style = {
    width: "50%",
    margin: "0 auto",
    marginTop: 150,
  };
  return (
    <div className="App">
      <div style={style}>
        <Sample />
      </div>
    </div>
  );
}

export default App;

npm start