Javascript 使用 React JS 进行下拉的 OnChange 事件

OnChange event using React JS for drop down

提问by user544079

var MySelect = React.createClass({
     change: function(){
         return document.querySelector('#lang').value;
     },
     render: function(){
        return(
           <div>
               <select id="lang">
                  <option value="select" onChange={this.change}>Select</option>
                  <option value="Java" onChange={this.change}>Java</option>
                  <option value="C++" onChange={this.change}>C++</option>
               </select>
               <p></p>
               <p value={this.change}></p>
           </div>
        );
     }
});

React.render(<MySelect />, document.body);

The onChangeevent does not work.

该onChange事件不起作用。

回答by Felix Kling

The change event is triggered on the <select>element, not the <option>element. However, that's not the only problem. The way you defined the changefunction won't cause a rerender of the component. It seems like you might not have fully grasped the concept of React yet, so maybe "Thinking in React"helps.

更改事件是在<select>元素上触发的，而不是在<option>元素上。然而，这并不是唯一的问题。您定义change函数的方式不会导致组件的重新渲染。看起来你可能还没有完全掌握 React 的概念，所以也许“Thinking in React”会有所帮助。

You have to store the selected value as state and update the state when the value changes. Updating the state will trigger a rerender of the component.

您必须将所选值存储为状态并在值更改时更新状态。更新状态将触发组件的重新渲染。

var MySelect = React.createClass({
     getInitialState: function() {
         return {
             value: 'select'
         }
     },
     change: function(event){
         this.setState({value: event.target.value});
     },
     render: function(){
        return(
           <div>
               <select id="lang" onChange={this.change} value={this.state.value}>
                  <option value="select">Select</option>
                  <option value="Java">Java</option>
                  <option value="C++">C++</option>
               </select>
               <p></p>
               <p>{this.state.value}</p>
           </div>
        );
     }
});

React.render(<MySelect />, document.body);

Also note that <p>elements don't have a valueattribute. React/JSX simply replicates the well-known HTML syntax, it doesn't introduce custom attributes (with the exception of keyand ref). If you want the selected value to be the content of the <p>element then simply put inside of it, like you would do with any static content.

另请注意，<p>元素没有value属性。React/JSX 只是简单地复制了众所周知的 HTML 语法，它没有引入自定义属性（除了key和ref）。如果您希望所选值成为<p>元素的内容，那么只需将其放入其中，就像处理任何静态内容一样。

Learn more about event handling, state and form controls:

了解有关事件处理、状态和表单控件的更多信息：

• http://facebook.github.io/react/docs/interactivity-and-dynamic-uis.html
• http://facebook.github.io/react/docs/forms.html

• http://facebook.github.io/react/docs/interactivity-and-dynamic-uis.html
• http://facebook.github.io/react/docs/forms.html

回答by Kirk Strobeck

import React, { PureComponent, Fragment } from 'react';
import ReactDOM from 'react-dom';

class Select extends PureComponent {
  state = {
    options: [
      {
        name: 'Select…',
        value: null,
      },
      {
        name: 'A',
        value: 'a',
      },
      {
        name: 'B',
        value: 'b',
      },
      {
        name: 'C',
        value: 'c',
      },
    ],
    value: '?',
  };

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

  render() {
    const { options, value } = this.state;

    return (
      <Fragment>
        <select onChange={this.handleChange} value={value}>
          {options.map(item => (
            <option key={item.value} value={item.value}>
              {item.name}
            </option>
          ))}
        </select>
        <h1>Favorite letter: {value}</h1>
      </Fragment>
    );
  }
}

ReactDOM.render(<Select />, window.document.body);

回答by ahota

React Hooks (16.8+):

反应钩子（16.8+）：

const Dropdown = ({
  options
}) => {
  const [selectedOption, setSelectedOption] = useState(options[0].value);
  return (
      <select
        value={selectedOption}
        onChange={e => setSelectedOption(e.target.value)}>
        {options.map(o => (
          <option value={o.value}>{o.label}</option>
        ))}
      </select>
  );
};

回答by Abolfazl Miadian

Thank you Felix Kling, but his answer need a little change:

谢谢 Felix Kling，但他的回答需要稍作改动：

var MySelect = React.createClass({
 getInitialState: function() {
     return {
         value: 'select'
     }
 },
 change: function(event){
     this.setState({value: event.target.value});
 },
 render: function(){
    return(
       <div>
           <select id="lang" onChange={this.change.bind(this)} value={this.state.value}>
              <option value="select">Select</option>
              <option value="Java">Java</option>
              <option value="C++">C++</option>
           </select>
           <p></p>
           <p>{this.state.value}</p>
       </div>
    );
 }
});
React.render(<MySelect />, document.body); 

回答by Satish

If you are using select as inline to other component, then you can also use like given below.

如果您使用 select 作为其他组件的内联，那么您也可以使用如下所示。

<select onChange={(val) => this.handlePeriodChange(val.target.value)} className="btn btn-sm btn-outline-secondary dropdown-toggle">
    <option value="TODAY">Today</option>
    <option value="THIS_WEEK" >This Week</option>
    <option value="THIS_MONTH">This Month</option>
    <option value="THIS_YEAR">This Year</option>
    <option selected value="LAST_AVAILABLE_DAY">Last Availabe NAV Day</option>
</select>

And on the component where select is used, define the function to handle onChange like below:

在使用 select 的组件上，定义处理 onChange 的函数，如下所示：

handlePeriodChange(selVal) {
    this.props.handlePeriodChange(selVal);
}