2019 https://moduscreate.com/blog/everything-you-need-to-know-about-refs-in-react/ https://rafaelquintanilha.com/the-complete-guide-to-react-refs/ 2018 https://css-tricks.com/working-with-refs-in-react/ https://proglib.io/p/react-js-interview/ render() {   var squareStyle = {     backgroundColor: this.state.bgColor   };     var self = this;     return (                               self._input = el; }                placeholder="Enter a color value">         go               ); } ... setNewColor(e) {   this.setState({     bgColor: this.state.color   });     this._input.focus();   this._input.value = "";     e.preventDefault(); }