DOM diffing, DOM shape, and mouse events (First version)

tooltip.es6.jsx

class HasTooltip extends React.Component {
  /* … */

  _renderPlaceholder() {
    return (
      <div className="has-tooltip"
        onMouseEnter={this._setHovered}
        onMouseLeave={this._setNotHovered}
        ></div>
    );
  }
  
  render() {
    const placeholder = this._renderPlaceholder();
    const {isHovered} = this.state;
    
    return isHovered ? (
      <div className="tooltip-region">
        { placeholder }
        <div>Hello</div>
      </div>
    ) : placeholder;
  }
}