gistfile1.js

  import cssStyle from './style'
  
  componentWillUnmount () {
    // delete the style when the component unmount
    let tag = document.querySelector('style[id="react-tooltip"]')
    document.getElementsByTagName('head')[0].removeChild(tag)
  }
  
  render() {
    // to check if the style has been existed
    if (!document.getElementsByTagName('head')[0].querySelector('style[id="react-tooltip"]')) {
      // insert the style into the head
      let tag = document.createElement('style')
      tag.id = 'react-tooltip'
      tag.innerHTML = cssStyle
      document.getElementsByTagName('head')[0].appendChild(tag)
    }
    return (...)
  }