React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}


button {
  width: 400px;
  display: block;
  margin-top: 2px;
}

React

class Headline extends React.PureComponent {
  render() {
   const {text} = this.props;
   return <h1>{text} (render time: {Date.now()})</h1>;
  }
} 

const simpleText = 'hello world'

const App = ()=> {
  const [text, setText] = React.useState(simpleText)
  const [object, setObject] = React.useState({text: simpleText})
  return (
    <div>
      <Headline text={text}/>
      <Headline text={object.text}/>

      <button onClick={()=>setText(simpleText)}>
        test 1: the first line should not change (expected)
      </button>

      <button onClick={()=>setObject({text: simpleText})}>
        test 2: the second line will not change  (expected)
      </button>

      <button onClick={()=>setText(new String(simpleText))}>
        test 3: the first line will change on every click (why?)
      </button>
    </div>
  )
}

ReactDOM.render(<App />, document.querySelector("#app"))