App.js

import React from 'react';

class App extends React.Component {
  constructor(){
    super();
    this.state = {expression: '0', runningTotal: 0 }
  }
  shouldComponentUpdate(nextProps) {
    //YOUR CODE GOES HERE
    return true;
  }
  update(strExpression){
    this.setState({expression: strExpression.target.value});
    try {
      this.setState({runningTotal: eval(strExpression.target.value), error: undefined});
    } catch(err) {
      this.setState({error: err});
      //ignore
    }
  }
  render(){
    return (
      <div>
        <input type="text" onKeyUp = {this.update.bind(this)} />
        <h1>{this.state.expression}={this.state.runningTotal}</h1>
      </div>
    );
  }
}

export default App