你应该在MyComponent extends React类下使用构造函数。组件

constructor(props){
    super(props);
    this.onSubmit = this.onSubmit.bind(this);
  }

然后你会得到标题的结果

使用不受控字段:

export default class MyComponent extends React.Component {

    onSubmit(e) {
        e.preventDefault();
        console.log(e.target.neededField.value);
    }

    render(){
        return (
            ...
            <form onSubmit={this.onSubmit} className="form-horizontal">
                ...
                <input type="text" name="neededField" className="form-control" ref={(c) => this.title = c}/>
                ...
            </form>
            ...
            <button type="button" className="btn">Save</button>
            ...
        );
    }

};

最简单的方法是使用箭头函数

箭头函数的代码

export default class MyComponent extends React.Component {

onSubmit = (e) => {
    e.preventDefault();
    var title = this.title;
    console.log(title);
}

render(){
    return (
        ...
        <form className="form-horizontal">
            ...
            <input type="text" className="form-control" ref={(c) => this.title = c} name="title" />
            ...
        </form>
        ...
        <button type="button" onClick={this.onSubmit} className="btn">Save</button>
        ...
    );
}

};

React版本:17.0.1 a)使用功能组件 b)使用hook: useState()管理状态。

编写并运行上面的代码:

import React, {useState} from 'react';

const InputElement = () => {
  const [inputText, setInputText] = useState('');

  return (
   <div> 
        <input
              onChange={(e) => {
                  setInputText(e.target.value);
                   }                   
             }
             placeholder='Enter Text'
       />
       {inputText}
   </div>
 );
}

求解方案算法类似于双向数据绑定:

input <=> DATA_MODEL <=> Label_Text

在功能组件中:-

export default function App(){

const [state, setState] = useState({
        value:'',
        show:''
    });

const handleChange = (e) => {
    setState({value: e.target.value})
}

const submit = () => {
    setState({show: state.value})
}

return(
        <>
            <form onSubmit={()=>submit()}>
                <input type="text" value={state.value} onChange={(e)=>handleChange(e)} />
                <input type="submit" />
            </form>
            <h2>{state.show}</h2>
        </>
)}

在react 16中，我使用

<Input id="number" 
       type="time" 
       onChange={(evt) => { console.log(evt.target.value); }} />