How handle multiple select form in ReactJS

Of my basic understanding, when you try to handle a Select form element in reactJS you generates an object in HTMLOptionsCollection.

The fundamental root to this object methods and properties is e.target.options.

Your items are stored in e.target.options.value property.

To access to a value stored in the options.value object, you can use the [i] loop value, hence e.target.options[i].value property.

The e.target.options[i].value return strings data types.

Following what I have just said, I assume the objects are stored respecting a number increasing convention as following :

e.target.options[i].value where { [i] : value, [i +1] : value (...)}...

By using e.target.options[i].selected you can control if there is a value stored at a specific location.

e.target.options[i].selected return you a boolean value, useful to handle the code flow.

It's up to you now.

Here my code to handle multiple select form in JSX with javascript code :

// Create the React Component


    class ChooseYourCharacter extends React.Component {

          // Set the constructor
          constructor(props) {
            super(props);
            this.state = {value: 'coconut'};

            // bind the functions
            this.handleChange = this.handleChange.bind(this);
            this.handleSubmit = this.handleSubmit.bind(this);
          }

          // extract the value to fluently setState the DOM
          handleChange (e) {
            var options = e.target.options;
            var value = [];
            for (var i = 0, l = options.length; i < l; i++) {
              if (options[i].selected) {
                value.push(options[i].value);
              }
            }
            this.setState({value: value});
          }

          // display in client-side the values choosen
          handleSubmit() {
             alert("you have choose :" + this.state.value);

         }


    (...)

Currently learning React and I noticed this same code on the reactjs.org site. Below is my solution for handling multiple selected options.

1. in the constructor, use an array for the initial value for 'value' in the state
2. in the handleChange method, convert the event target's selectedOptions (HTMLOptionsCollection - array-like) to an array using Array.from(), and use a mapping function to get the value from each item

class ChooseYourCharacter extends React.Component {

      constructor(props) {
        super(props);
        //this.state = {value: 'coconut'};
        this.state = {value: ['coconut']};

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

      handleChange(event) {
        //this.setState({value: event.option});
        this.setState({value: Array.from(event.target.selectedOptions, (item) => item.value)});
      }

      handleSubmit(event) {
        alert('Your favorite flavor is: ' + this.state.value);
        event.preventDefault();
      }

      render() {
        return (
          <form onSubmit={this.handleSubmit}>
            <label>
              Pick your favorite La Croix flavor:
              <select multiple={true} value={this.state.value} onChange={this.handleChange}>
                <option value="grapefruit">Grapefruit</option>
                <option value="lime">Lime</option>
                <option value="coconut">Coconut</option>
                <option value="mango">Mango</option>
              </select>
            </label>
            <input type="submit" value="Submit" />
          </form>
        );
      }
    }
    ReactDOM.render(
             <ChooseYourCharacter/>,
             document.getElementById('root')
    )

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
<div id="root"></div>

Here is how to get the options selected by the user using a functional component and the useState hook rather than a class component:

import React, { useState } from "react";

const ChooseYourCharacter = function(props) {
    const [selectedFlavors, setSelectedFlavors] = useState([]);

    const handleSelect = function(selectedItems) {
        const flavors = [];
        for (let i=0; i<selectedItems.length; i++) {
            flavors.push(selectedItems[i].value);
        }
        setSelectedFlavors(flavors);
    }

    return (
        <form>
            <select multiple={true} value={selectedFlavors} onChange={(e)=> {handleSelect(e.target.selectedOptions)}}>
                <option value="grapefruit">Grapefruit</option>
                <option value="lime">Lime</option>
                <option value="coconut">Coconut</option>
                <option value="mango">Mango</option>
            </select>
        </form>
    );
};

export default ChooseYourCharacter;