如何在 ReactJS 中获得下拉菜单的选定值

The code in the render method represents the component at any given time. If you do something like this, the user won't be able to make selections using the form control:

<select value="Radish">
<option value="Orange">Orange</option>
<option value="Radish">Radish</option>
<option value="Cherry">Cherry</option>
</select>

So there are two solutions for working with forms controls:

1. Controlled Components Use component state to reflect the user's selections. This provides the most control, since any changes you make to state will be reflected in the component's rendering:

example:

var FruitSelector = React.createClass({
getInitialState:function(){
return {selectValue:'Radish'};
},
handleChange:function(e){
this.setState({selectValue:e.target.value});
},
render: function() {
var message='You selected '+this.state.selectValue;
return (
<div>
<select
value={this.state.selectValue}
onChange={this.handleChange}
>
<option value="Orange">Orange</option>
<option value="Radish">Radish</option>
<option value="Cherry">Cherry</option>
</select>
<p>{message}</p>
</div>
);
}
});


React.render(<FruitSelector name="World" />, document.body);

JSFiddle: http://jsfiddle.net/xe5ypghv/

2. Uncontrolled Components The other option is to not control the value and simply respond to onChange events. In this case you can use the defaultValue prop to set an initial value.

   <div>
   <select defaultValue={this.state.selectValue}
   onChange={this.handleChange}
   >
   <option value="Orange">Orange</option>
   <option value="Radish">Radish</option>
   <option value="Cherry">Cherry</option>
   </select>
   <p>{message}</p>
   </div>
   

http://jsfiddle.net/kb3gN/10396/

The docs for this are great: http://facebook.github.io/react/docs/forms.html and also show how to work with multiple selections.

UPDATE

A variant of Option 1 (using a controlled component) is to use Redux and React-Redux to create a container component. This involves connect and a mapStateToProps function, which is easier than it sounds but probably overkill if you're just starting out.

Just use onChange event of the <select> object. Selected value is in e.target.value then.

By the way, it's a bad practice to use id="...". It's better to use ref=">.." http://facebook.github.io/react/docs/more-about-refs.html

Implement your Dropdown as

<select id = "dropdown" ref = {(input)=> this.menu = input}>
<option value="N/A">N/A</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>

Now, to obtain the selected option value of the dropdown menu just use:

let res = this.menu.value;

As for front-end developer many time we are dealing with the forms in which we have to handle the dropdowns and we have to use the value of selected dropdown to perform some action or the send the value on the Server, it's very simple you have to write the simple dropdown in HTML just put the one onChange method for the selection in the dropdown whenever user change the value of dropdown set that value to state so you can easily access it in AvFeaturedPlayList 1 remember you will always get the result as option value and not the dropdown text which is displayed on the screen

import React, { Component } from "react";
import { Server } from "net";


class InlineStyle extends Component {
constructor(props) {
super(props);
this.state = {
selectValue: ""
};


this.handleDropdownChange = this.handleDropdownChange.bind(this);
}


handleDropdownChange(e) {
this.setState({ selectValue: e.target.value });
}


render() {
return (
<div>
<div>
<div>
<select id="dropdown" onChange={this.handleDropdownChange}>
<option value="N/A">N/A</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
</div>


<div>Selected value is : {this.state.selectValue}</div>
</div>
</div>
);
}
}
export default InlineStyle;

import React from 'react';
import Select from 'react-select';


const options = [
{ value: 'chocolate', label: 'Chocolate' },
{ value: 'strawberry', label: 'Strawberry' },
{ value: 'vanilla', label: 'Vanilla' },
];


class App extends React.Component {
state = {
selectedOption: null,
};
handleChange = selectedOption => {
this.setState({ selectedOption });
console.log(`Option selected:`, selectedOption);
};
render() {
const { selectedOption } = this.state;


return (
<Select
value={selectedOption}
onChange={this.handleChange}
options={options}
/>
);
}
}

And you can check it out on this site.

You can handle it all within the same function as following

<select className="form-control mb-3" onChange={(e) => this.setState({productPrice: e.target.value})}>


<option value="5">5 dollars</option>
<option value="10">10 dollars</option>
                                         

</select>

as you can see when the user select one option it will set a state and get the value of the selected event without furder coding require!

It is as simple as that. You just need to use "value" attributes instead of "defaultValue" or you can keep both if a pre-selected feature is there.

 ....
const [currentValue, setCurrentValue] = useState(2);
<select id = "dropdown" value={currentValue} defaultValue={currentValue}>
<option value="N/A">N/A</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>
.....


setTimeut(()=> {
setCurrentValue(4);
}, 4000);

In this case, after 4 secs the dropdown will be auto-selected with option 4.

I was making a drop-down menu for a language selector - but I needed the dropdown menu to display the current language upon page load. I would either be getting my initial language from a URL param example.com?user_language=fr, or detecting it from the user’s browser settings. Then when the user interacted with the dropdown, the selected language would be updated and the language selector dropdown would display the currently selected language.

In the spirit of the other answers using food examples, I got all sorts of fruit goodness for you.

• First up, answering the initially asked question with a basic React functional component - two examples with and without props, then how to import the component elsewhere.

• Next up, the same example - but juiced up with Typescript.

• Then a bonus finale - A language selector dropdown component using Typescript.

Basic React (16.13.1) Functional Component Example. Two examples of FruitSelectDropdown , one without props & one with accepting props fruitDetector

import React, { useState } from 'react'


export const FruitSelectDropdown = () => {
const [currentFruit, setCurrentFruit] = useState('oranges')
  

const changeFruit = (newFruit) => {
setCurrentFruit(newFruit)
}
  

return (
<form>
<select
onChange={(event) => changeFruit(event.target.value)}
value={currentFruit}
>
<option value="apples">Red Apples</option>
<option value="oranges">Outrageous Oranges</option>
<option value="tomatoes">Technically a Fruit Tomatoes</option>
<option value="bananas">Bodacious Bananas</option>
</select>
</form>
)
}

Or you can have FruitSelectDropdown accept props, maybe you have a function that outputs a string, you can pass it through using the fruitDetector prop

import React, { useState } from 'react'


export const FruitSelectDropdown = ({ fruitDetector }) => {
const [currentFruit, setCurrentFruit] = useState(fruitDetector)
  

const changeFruit = (newFruit) => {
setCurrentFruit(newFruit)
}
  

return (
<form>
<select
onChange={(event) => changeFruit(event.target.value)}
value={currentFruit}
>
<option value="apples">Red Apples</option>
<option value="oranges">Outrageous Oranges</option>
<option value="tomatoes">Technically a Fruit Tomatoes</option>
<option value="bananas">Bodacious Bananas</option>
</select>
</form>
)
}

Then import the FruitSelectDropdown elsewhere in your app

import React from 'react'
import { FruitSelectDropdown } from '../path/to/FruitSelectDropdown'


const App = () => {
return (
<div className="page-container">
<h1 className="header">A webpage about fruit</h1>
<div className="section-container">
<h2>Pick your favorite fruit</h2>
<FruitSelectDropdown fruitDetector='bananas' />


</div>
</div>
)
}


export default App

FruitSelectDropdown with Typescript

import React, { FC, useState } from 'react'


type FruitProps = {
fruitDetector: string;
}


export const FruitSelectDropdown: FC<FruitProps> = ({ fruitDetector }) => {
const [currentFruit, setCurrentFruit] = useState(fruitDetector)
  

const changeFruit = (newFruit: string): void => {
setCurrentFruit(newFruit)
}
  

return (
<form>
<select
onChange={(event) => changeFruit(event.target.value)}
value={currentFruit}
>
<option value="apples">Red Apples</option>
<option value="oranges">Outrageous Oranges</option>
<option value="tomatoes">Technically a Fruit Tomatoes</option>
<option value="bananas">Bodacious Bananas</option>
</select>
</form>
)
}

Then import the FruitSelectDropdown elsewhere in your app

import React, { FC } from 'react'
import { FruitSelectDropdown } from '../path/to/FruitSelectDropdown'


const App: FC = () => {
return (
<div className="page-container">
<h1 className="header">A webpage about fruit</h1>
<div className="section-container">
<h2>Pick your favorite fruit</h2>
<FruitSelectDropdown fruitDetector='bananas' />


</div>
</div>
)
}


export default App

Bonus Round: Translation Dropdown with selected current value:

import React, { FC, useState } from 'react'
import { useTranslation } from 'react-i18next'


export const LanguageSelectDropdown: FC = () => {
const { i18n } = useTranslation()
const i18nLanguage = i18n.language
const [currentI18nLanguage, setCurrentI18nLanguage] = useState(i18nLanguage)
  

const changeLanguage = (language: string): void => {
i18n.changeLanguage(language)
setCurrentI18nLanguage(language)
}
  

return (
<form>
<select
onChange={(event) => changeLanguage(event.target.value)}
value={currentI18nLanguage}
>
<option value="en">English</option>
<option value="de">Deutsch</option>
<option value="es">Español</option>
<option value="fr">Français</option>
</select>
</form>
)
}

An invaluable resource for React/Typescript

Using React Functional Components:

const [option,setOption] = useState()


function handleChange(event){
setOption(event.target.value)
}


<select name='option' onChange={handleChange}>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>

If you want to get value from a mapped select input then you can refer to this example:

class App extends React.Component {
constructor(props) {
super(props);
this.state = {
fruit: "banana",
};
    

this.handleChange = this.handleChange.bind(this);
}
    

handleChange(e) {
console.log("Fruit Selected!!");
this.setState({ fruit: e.target.value });
}
    

render() {
return (
<div id="App">
<div className="select-container">
<select value={this.state.fruit} onChange={this.handleChange}>
{options.map((option) => (
<option value={option.value}>{option.label}</option>
))}
</select>
</div>
</div>
);
}
}
    

export default App;

It should be like:

import React, { useState } from "react";


export default function App() {
const getInitialState = () => {
const value = "Orange";
return value;
};


const [value, setValue] = useState(getInitialState);


const handleChange = (e) => {
setValue(e.target.value);
};


return (
<div>
<select value={value} onChange={handleChange}>
<option value="Orange">Orange</option>
<option value="Radish">Radish</option>
<option value="Cherry">Cherry</option>
</select>
<p>{`You selected ${value}`}</p>
</div>
);
}

you can see it here: https://codesandbox.io/s/quizzical-https-t1ovo?file=/src/App.js:0-572

import {React, useState }from "react";


function DropDown() {


const [dropValue, setDropValue ]= useState();


return <>
<div>
<div class="dropdown">
<button class="btn btn-secondary" type="button" id="dropdownMenuButton1" data-bs-toggle="dropdown" aria-expanded="false">
{dropValue==null || dropValue=='' ?'Select Id':dropValue}
</button>
<ul class="dropdown-menu" aria-labelledby="dropdownMenuButton1">
<li><a class="dropdown-item" onClick={()=> setDropValue('Action')} href="#">Action</a></li>
<li><a class="dropdown-item" onClick={()=> setDropValue('Another action')} href="#">Another action</a></li>
<li><a class="dropdown-item" onClick={()=> setDropValue('Something else here')} href="#">Something else here</a></li>
</ul>
</div>
</div>
</>
}


export default DropDown

<select value ={this.state.value} onChange={this.handleDropdownChange}>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>

As mentioned by Karen above you can just use the target value from the event triggered. Here is a small snippet of the code

                 `<select class="form-select py-2"
onChange={(e) => setVotersPerPage(e.target.value)}>
<option value="10">10</option>
<option value="25">25</option>
<option value="50">50</option>
</select>`