为什么输入在 React 中不返回任何内容？

Question

我是 React 的初学者，我正在尝试用它制作小应用程序。 事实上，这是我在 React 中制作的第一个应用程序，但我遇到了一个问题。

我正在制作一个小型应用程序，可以输入数字并对其进行加、减或乘运算，即计算器。 这是代码。

 const { Component } = React; class App extends React.Component { state = { num1: 0, num2: 0, answer: 0, equation: "", }; handleAdd1 = (number) => { this.setState({ num1: number }); }; handleAdd2 = (number2) => { this.setState({ num2: number2 }); }; handleClear = () => { this.setState({ num1: 0, num2: 0, answer: 0 }); }; handlePlus = (sign) => { switch (sign) { case "+": this.setState({ answer: this.state.num1 + this.state.num2 }); break; case "-": this.setState({ answer: this.state.num1 - this.state.num2 }); break; case "x": this.setState({ answer: this.state.num1 * this.state.num2 }); break; } }; handleInput = (event) => { this.setState({ equation: event.target.value }); }; render() { return ( <div> <div> <h1>{this.state.num1}</h1> <button onClick={() => this.handleAdd1(1)}> 1 </button> <button onClick={() => this.handleAdd1(2)}> 2 </button> <button onClick={() => this.handleAdd1(3)}> 3 </button> <button onClick={() => this.handleAdd1(4)}> 4 </button> <button onClick={() => this.handleAdd1(5)}> 5 </button> <button onClick={() => this.handleAdd1(6)}> 6 </button> <button onClick={() => this.handleAdd1(7)}> 7 </button> <button onClick={() => this.handleAdd1(8)}> 8 </button> <button onClick={() => this.handleAdd1(9)}> 9 </button> <button onClick={() => this.handleAdd1(0)}> 0 </button> </div> <div> <h1>{this.state.num2}</h1> <button onClick={() => this.handleAdd2(1)}> 1 </button> <button onClick={() => this.handleAdd2(2)}> 2 </button> <button onClick={() => this.handleAdd2(3)}> 3 </button> <button onClick={() => this.handleAdd2(4)}> 4 </button> <button onClick={() => this.handleAdd2(5)}> 5 </button> <button onClick={() => this.handleAdd2(6)}> 6 </button> <button onClick={() => this.handleAdd2(7)}> 7 </button> <button onClick={() => this.handleAdd2(8)}> 8 </button> <button onClick={() => this.handleAdd2(9)}> 9 </button> <button onClick={() => this.handleAdd2(0)}> 0 </button> </div> <div> <button onClick={() => this.handlePlus("+")}>+</button> <button onClick={() => this.handlePlus("-")}>-</button> <button onClick={() => this.handlePlus("x")}>x</button> </div> <div> <button onClick={this.handleClear}>Clear</button> <h1>{this.state.answer}</h1> <input type="text"></input> <button onClick={() => this.handleInput}>Enter !</button> <button onClick={() => console.log(this.state.equation)}> Log in console </button> </div> </div> ); } } ReactDOM.render(<App />, document.body);

 <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>

如果你查看程序，你会看到有几个数字，如果你点击它们并添加，你会得到一个 h1 的结果。 但是，这并不是很好，因为您只能添加个位数。 因此，如您所见，我添加了一个输入，它将获取等式并将其记录在控制台中（暂时）。 但遗憾的是，这是返回未定义。 我能做些什么来解决这个问题吗？

Answer 1

您需要使输入成为受控组件。 您可以通过监听onChange事件并将值传递给 value 属性来制作输入控制组件

<input type="text" value={this.state.equation} onChange={(e)=>this.setState({equation: e.target.value})}

现在你将得到等式

Answer 2

您忘记映射输入标签的值，您可以将其与状态绑定，如果您不熟悉反应，您应该开始使用钩子，这是现在更清晰的方法，
这是完整的工作解决方案，
https://codesandbox.io/embed/inspiring-tesla-9sjop?fontsize=14&hidenavigation=1&theme=dark

import React, { Component } from "react";

class App extends React.Component {
  state = {
    num1: 0,
    num2: 0,
    answer: 0,
    equation: "",
    value: ""
  };

  handleChange = (e) => {
    this.setState({ equation: e.target.value });
  };

  handleAdd1 = (number) => {
    this.setState({ num1: number });
  };

  handleAdd2 = (number2) => {
    this.setState({ num2: number2 });
  };

  handleClear = () => {
    this.setState({ num1: 0, num2: 0, answer: 0 });
  };

  handlePlus = (sign) => {
    switch (sign) {
      case "+":
        this.setState({ answer: this.state.num1 + this.state.num2 });
        break;

      case "-":
        this.setState({ answer: this.state.num1 - this.state.num2 });
        break;

      case "x":
        this.setState({ answer: this.state.num1 * this.state.num2 });
        break;
    }
  };

  handleInput = (event) => {
    this.setState({ equation: event.target.value });
  };

  render() {
    return (
      <div>
        <div>
          <h1>{this.state.num1}</h1>
          <button onClick={() => this.handleAdd1(1)}> 1 </button>
          <button onClick={() => this.handleAdd1(2)}> 2 </button>
          <button onClick={() => this.handleAdd1(3)}> 3 </button>
          <button onClick={() => this.handleAdd1(4)}> 4 </button>
          <button onClick={() => this.handleAdd1(5)}> 5 </button>
          <button onClick={() => this.handleAdd1(6)}> 6 </button>
          <button onClick={() => this.handleAdd1(7)}> 7 </button>
          <button onClick={() => this.handleAdd1(8)}> 8 </button>
          <button onClick={() => this.handleAdd1(9)}> 9 </button>
          <button onClick={() => this.handleAdd1(0)}> 0 </button>
        </div>

        <div>
          <h1>{this.state.num2}</h1>
          <button onClick={() => this.handleAdd2(1)}> 1 </button>
          <button onClick={() => this.handleAdd2(2)}> 2 </button>
          <button onClick={() => this.handleAdd2(3)}> 3 </button>
          <button onClick={() => this.handleAdd2(4)}> 4 </button>
          <button onClick={() => this.handleAdd2(5)}> 5 </button>
          <button onClick={() => this.handleAdd2(6)}> 6 </button>
          <button onClick={() => this.handleAdd2(7)}> 7 </button>
          <button onClick={() => this.handleAdd2(8)}> 8 </button>
          <button onClick={() => this.handleAdd2(9)}> 9 </button>
          <button onClick={() => this.handleAdd2(0)}> 0 </button>
        </div>

        <div>
          <button onClick={() => this.handlePlus("+")}>+</button>
          <button onClick={() => this.handlePlus("-")}>-</button>
          <button onClick={() => this.handlePlus("x")}>x</button>
        </div>

        <div>
          <button onClick={this.handleClear}>Clear</button>
          <h1>{this.state.answer}</h1>
          <input
            type="text"
            value={this.state.equation}
            onChange={this.handleChange}
          ></input>
          <button onClick={() => this.handleInput}>Enter !</button>
          <button onClick={() => console.log(this.state.equation)}>
            Log in console
          </button>
        </div>
      </div>
    );
  }
}

export default App;