<-- /notes<-- /notes/react-101

Component Props Objects

While we learned earlier that props cannot be modified, we can customize the ways in which we get our props to our component. There are certain objects which we can attach after we've built our component which can help us use props properly.

Default Props

Sometimes, we'd like to have certain properties be the default unless specified by the user. We do so by using the following:

class Example extends React.Component {
  render() {
    return <button>{this.props.msg}</button>

Example.defaultProps = {
  msg: 'I am a button.';

  Rendering <Example /> will have a msg value of 'I am a button.'
  Rendering <Example msg='' /> will have a msg value of ''

Prop Types

The more React Components we use and create, the easier it is to forget what type of value it needs. This kind of thing can get mixed up easily depending on the amount of people working with your code. That's why we use the propTypes object to prevent any bad input from affecting our components. Take a look at this example.

const SoftwareStatus = (props) => {
  return (
      <h2>{props.isUpdated ? 'Up-to-date ' + props.versionNumber : 'Requires Update'}</h2>
      <img src={props.thumbnail} alt={props.program + ' thumbnail'}>

Here we have a SFC which is completely dependent on having props in order to display properly. In order to insure it is passed the correct and required information, we do the following:

SoftwareStatus.propTypes = {
  program         = React.PropTypes.string.isRequired,
  isUpdated       = React.PropTypes.bool.isRequired,
  versionNumber   = React.PropTypes.number,
  thumbnail       = React.PropTypes.string.isRequired

Taking a look at the propTypes object, we can see we put requirements on every instance of a SoftwareStatus component. Using React.PropTypes.* allows us to force certain data types when we create this component, and .isRequired requires that every instance possesses the attached prop. Here are some examples:

  {/* INVALID - isUpdated requires a boolean, versionNumber requires a number */}
  {/* INVALID - program is a required field */}
  {/* VALID */}

There is also the ability to specify the shape of the data structure that is passed to the React component, to ensure the correct data is passed, and TypeErrors are less likely.

DataCollector.propTypes = {
  data: PropTypes.shape({
    name: PropTypes.string,
    age: PropTypes.number,
  operators: PropTypes.arrayOf(PropTypes.number),
  title: PropTypes.string,

You can check out the full docs over here.

Component Control

Any given component can be considered a controlled component, or an uncontrolled component. In a quick explanation, A controlled component does not maintain an internal state, while a uncontrolled component does. In other words, an uncontrolled component is uncontrollable and vice versa. 99% of the time, components are controlled, because React works best with these. Let's take a look at an example:

import React from 'react';
import ReactDOM from 'react-dom';

class Display extends React.Component {
  constructor(props) {
    this.state = { userInput: '' };
    this.handleUserInput = this.handleUserInput.bind(this);
  handleUserInput(e) {
      userInput = e.target.value;
  render() {
        <input type="text" onChange={this.handleUserInput} value={this.state.userInput}/>
        <h1>This is your input: {this.state.userInput}</h1>

ReactDOM.render(<Display />, document.getElementById('app'));

For a quick TL;DR of this code, we type x into <input />, <Display />'s state userInput is set to x, it renders this.state.userInput. Simple enough.

But actually there's something going on behind the scene. <input /> is normally an uncontrolled component. This is because if we ever query for input's value (ex. document.querySelector('input[type="text"]').value), we would get it's value without having to consult <Display />'s state. By forcing the prop value={this.state.userInput} onto the <input /> tag, we convert it to a controlled element!