React: PropTypes

Learning Goals

• Implement PropTypes and explain why they are useful

Vocabulary

• PropTypes a library we use to validate the data type of props coming into a component; allows for more specific, helpful error messages

git clone git@github.com:turingschool-examples/react-iii-ideabox.git
cd react-iii-ideabox
npm i
npm start

git clone https://github.com/turingschool-examples/ideabox-api.git ideabox-api
cd ideabox-api
npm i
node server.js

PropTypes

PropTypes allow you to specify what type of props you are expecting in a certain component. This is also known as “typechecking”.

Many people have moved to implementing languages like TypeScript or Flow for this exact purpose, but even without any additional technologies, the prop-type module lets you establish a safety net with very little effort.

Let’s say you declare a component <Card title={title}/>. Here, your component is expecting a prop called title and you (probably) expect it to be a string. If you define that value within your propTypes object and it comes in as something else - say for example the API you are consuming has changed and now you have an array of strings - you will get a helpful warning message in your console.

Previously, PropTypes was built into the React library itself, however it has since been extracted into its own module. You can install it like any other module.

$ npm install prop-types

In React, PropTypes are declared like this:

// Card.js

import React from 'react';
import PropTypes from 'prop-types';
import './Card.css';

const Card = ({ id, title, description, removeIdea, isFavorite }) => {
  const favoriteClass = isFavorite ? 'favorite' : 'card';

  return (
    <section className={favoriteClass}>
      <h3>{ title }</h3>
      <p>{ description }</p>
      <button onClick={() => removeIdea(id)}>🗑</button>
    </section>
  );
};

export default Card;

Card.propTypes = {
  title: PropTypes.string
};

The error you will see if the component gets something besides a string would look something like this:

Warning: Failed prop type: Invalid prop `name` of type `array` supplied to `Card`, expected `string`.

Without using PropTypes, you would have seen a vague error when the Component failed to render. With PropTypes, we can see that the error was that we were receiving an array when we expected a string.

Class propTypes

Check out a complete list of propTypes here. Note: The usage in this example is deprecated, but the list of propTypes is still valid. Checkout a modern usage example here.

By default, all props specified within the Class.propTypes object will be considered optional. There are many instances where your component will not function correctly without that particular prop. To add a validation that will fire an error message if a prop does not show up at all, simply add .isRequired to the end of the propType declaration.

Card.propTypes = {
  title: PropTypes.string.isRequired
};

You can also be more generic - let’s say you need a prop to come in but it doesn’t matter what type it is as long as it’s there. Instead of specifying a particular JS primitive you can use .any.

Card.propTypes = {
  title: PropTypes.any.isRequired
};

DefaultProps

Just like when writing functions, React also allows us to provide a default value for props. defaultProps let you ensure that a value will be passed through. This helps eliminate some of the incessant ternaries that either render the prop or an empty string, for instance. Note: the above example uses a class component instead of a functional component, but the actual propTypes implementation can be the same.

const Card = ({ id, title, description, removeIdea, isFavorite }) => {
  const favoriteClass = isFavorite ? 'favorite' : 'card'

  return (
    <section className={favoriteClass}>
      <h3>{ title }</h3>
      <p>{ description }</p>
      <button onClick={() => removeIdea(id)}>🗑</button>
    </section>
  );
};

export default Card;

Card.propTypes = {
  id: PropTypes.number.isRequired,
  title: PropTypes.string.isRequired,
  description: PropTypes.string.isRequired,
  removeIdea: PropTypes.func.isRequired,
  isFavorite: PropTypes.bool.isRequired,
};

Card.defaultProps = {
  isFavorite: true
};

Extra Reading on PropTypes

Now that we’ve talked about the most obvious use cases of propTypes to preemptively debug your code, read the following article - you are highly encouraged to take notes:

• Better Prop Validation.