How to use debounce in ReactJS

Question

I'm learning ReactJS and stuck with the following problem. I have an input for contact searching and want to handle it in 1000ms after a user stop typing. I use debounce function for this purpose:

import React, { Component}  from 'react';
import ReactDOM  from 'react-dom';
import './index.css';
import {debounce} from 'lodash';

const contacts = [
    {
        id: 1,
        name: 'Darth Vader',
        phoneNumber: '[phone]',
        image: 'img/darth.gif'
    }, {
        id: 2,
        name: 'Princess Leia',
        phoneNumber: '[phone]',
        image: 'img/leia.gif'
    }, {
        id: 3,
        name: 'Luke Skywalker',
        phoneNumber: '[phone]',
        image: 'img/luke.gif'
    }, {
        id: 4,
        name: 'Chewbacca',
        phoneNumber: '[phone]',
        image: 'img/chewbacca.gif'
    }
];

class Contact extends React.Component {
    render() {
        return (
            <li className="contact">
                <img className="contact-image" src={this.props.image} width="60px" height="60px"/>
                <div className="contact-info">
                    <div className="contact-name">{this.props.name}</div>
                    <div className="contact-number">{this.props.phoneNumber}</div>
                </div>
            </li>
        );
    }
}

class ContactList extends React.Component {
    constructor(props) {
        super(props);
        this.state = {
            displayedContacts: contacts,
        };
        this.handleChange = debounce(this.handleChange.bind(this), 1000);
    }

    handleChange = e => {
        e.persist();
        let input = e.target.value.toLowerCase();
        this.setState({
            displayedContacts: contacts.filter(c => c.name.toLowerCase().includes(input))
        });   
    }

    render() {
        return (
            <div className="contacts">
                <input type="text" className="search-field" onChange={this.handleChange}/>
                <ul className="contacts-list">
                    {
                        this.state.displayedContacts.map(c =>
                            <Contact 
                                key={c.id} 
                                name={c.name}
                                phoneNumber={c.phoneNumber}
                                image={c.image} />
                        )
                    }
                </ul>
            </div>
        );
    }
}

ReactDOM.render(
    <ContactList />,
    document.getElementById('root')
);

In console log after filling in the search input I get a warning "This synthetic event is reused for performance reasons. If you're seeing this, you're accessing the property target on a released/nullified synthetic event. This is set to null. If you must keep the original synthetic event around, use event.persist()"

and an error "Uncaught TypeError: Cannot read property 'value' of null at ContactList._this2.handleChange".

I use persist method in handleChange function. Why I get this error?

Answer 1

Events handlers are best run synchronously. You could handle the value separately and filter the contacts in a separate debounced function.

Example

class ContactList extends React.Component {
  state = {
    contacts,
    displayedContacts: contacts
  };

  setDisplayedContacts = debounce(query => {
    this.setState({
      displayedContacts: this.state.contacts.filter(c =>
        c.name.toLowerCase().includes(query)
      )
    });
  }, 1000);

  handleChange = e => {
    let input = e.target.value.toLowerCase();
    this.setDisplayedContacts(input);
  };

  render() {
    return (
      <div className="contacts">
        <input
          type="text"
          className="search-field"
          onChange={this.handleChange}
        />
        <ul className="contacts-list">
          {this.state.displayedContacts.map(c => (
            <Contact
              key={c.id}
              name={c.name}
              phoneNumber={c.phoneNumber}
              image={c.image}
            />
          ))}
        </ul>
      </div>
    );
  }
}