Component.js

// Preview.js
let Preview = (props) => {
  const {username, tex} = props;
  
  return (
    <div>{username}</div>
    <div>$${tex}$$</div>
  )
}

Preview.componentDidMount = function() {
  MathJax.Hub.Queue(["Typeset",MathJax.Hub]);
}

// Index.js
ReactDOM.render(
  <Provider store={store}>
    <MuiThemeProvider muiTheme={getMuiTheme()}>
      <div style={{ display: 'flex', flexDirection:'row' }}>
        <div>
          <Form onSubmit={showResults} />
        </div>
        <div>
          <Preview />
        </div>
      </div>
    </MuiThemeProvider>
  </Provider>,
  document.getElementById('root'));