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;
}


h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.text {
  width: 100%;
  padding: 3px;
  font-size: 16px;
  box-sizing: border-box;
  margin-top: 1rem;
}

.output {
  font-size: 15px;
  color: #444;
  margin-top: 1rem
}

React

class Weirdness extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
    	stuff: ''
    }
    this.onChange = this.onChange.bind(this);
  }
  
  onChange(e) {
  	this.setState({
    	stuff: e.target.value
    });
    console.log('[Length of Stuff is]:', this.state.stuff.length);
  }
  
  render() {
    return (
      <div>
        <h2>Weird length reporting in the console</h2>
        <i style={{fontSize: "12px"}}>Open the console and type a word. The rendered count is shown below and is what you expect. Now hit the delete key to watch the length count INCREASE in the console!</i>
        
        <input 
          type="text"
          className="text"
          name="stuff"
          placeholder="Type a word and watch the weirdness"
          onChange={this.onChange}
          value={this.state.stuff}
         >       
        </input>
        
        <div className="output">The rendered length of stuff is : {this.state.stuff.length}</div>
      </div>
    );
  }
}

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