React's PropTypes is a powerful tool for developers to use when building their projects. It can be used to validate the data that a component receives as input. And ensure that it is what was expected.

The PropTypes module contains a set of functions for validating the types of data passed in as props. And return errors when validation fails. The most common use case is testing that required props are provided by children components.

How PropTypes Work

PropTypes is React's internal mechanism to check if the component is receiving correct types. React components use a property called propTypes to set up type checking. This can be done for both functional and class-based components.

import React from "react";
import PropTypes from "prop-types";

const Person = ({ name, age }) => {
  return (
    <article>
      <h1>{name}</h1>
      <p>{age}</p>
    </article>
  );
};

Person.propTypes = {
  name: PropTypes.string,
  age: PropTypes.number,
};

export default Person;

Available Types to Check

When it comes to supported types. PropTypes package offers predefined validation functions. They cover most of the usual types. And for more difficult validations, we also have the ability to define custom validators.

Basic Types

These are the validators for basic data types:

• PropTypes.any - prop can be anything
• PropTypes.bool - prop needs to be true or false
• PropTypes.number - prop needs to be any number
• PropTypes.string - prop needs to be any string
• PropTypes.func - prop needs to be a function
• PropTypes.array - prop needs to be Array
• PropTypes.object - prop needs to be Object
• PropTypes.symbol - prop needs to be Symbol

Person.propTypes = {
  hobby: PropTypes.any,
  male: PropTypes.bool,
  age: PropTypes.number,
  name: PropTypes.string,
  secretTalent: PropTypes.func,
  friends: PropTypes.array,
  wallet: PropTypes.object,
  skill: PropTypes.symbol,
};

Renderable Types

Renderable types check if prop can be rendered by React. We can specify if prop is React element(ie. <MyComponent />) or anything else(ie. strings, numbers, etc...)

• PropTypes.element - React element
• PropTypes.node - Anything that can be rendered. Numbers, strings, elements, or an array

Blog.propTypes = {
  blogTitle: PropTypes.element,
  blogBody: PropTypes.node,
};

Instance Types

An instance validator can be used to check if prop is an instance of the given class. This can be useful to check if prop is an instance of the component, but it only works with class-based components.

Collection Types

Collection validators come in handy when we need to validate the content of an array. This lets us check and validate the contents of the arrays or objects.

• PropTypes.arrayOf - Array of defined elements
• PropTypes.shape - Object containing defined properties(may contain additional properties)
• PropTypes.exact - Object containing only defined properties(can't contain any additional properties)

Person.propTypes = {
  friends: PropTypes.arrayOf(PropTypes.string),
  father: PropTypes.shape({
    name: PropTypes.string,
    age: PropTypes.number,
  }),
  dog: PropTypes.exact({
    name: PropTypes.string,
    age: PropTypes.number,
  }),
};

Required Types

All props are optional by default. To ensure that certain prop is always passed. We can mark it as required by using isRequired validator.

Person.propTypes = {
  name: PropTypes.string.isRequired,
  age: PropTypes.number.isRequired,
  friends: PropTypes.arrayOf(PropTypes.string),
};

Custom Validators

In some cases, we need to write custom validators. A good example of this is checking if passed prop is a valid email address.

We can define a custom validator as a function that takes 3 arguments and return Error when validation fails.

Person.propTypes = {
  email: (props, propName, componentName) => {
    // Regex to test if email is correct
    if (!/^[^\s@]+@[^\s@]+$/.test(props[propName])) {
      return new Error(
        `Invalid prop "${propsName}" supplied to "${componentName}"`
      );
    }
  },
};

Providing Default Values

It's simple to define default values for component props. We can do this by assigning them to defaultProps property. The following example will set englishSpeaker to true if we forget to set the prop.

Person.propTypes = {
  englishSpeaker: PropTypes.bool,
};

Person.defaultProps = {
  englishSpeaker: true,
};

How to Check for Errors

React's PropTypes check if the types are correct during runtime. This means we need to run our application to check if the types are correct. Additionally, it only checks the types of components that are rendered. If some component received an incorrect type. We'll see a warning in the console.

As this is a time-consuming task. It is only supported in development mode. So look at the console carefully while developing. Because you won't see a warning in production!

Conclusion

As the project grows larger, it's easy to miss warnings in the console. And introduce unwanted bugs into the system. In these scenarios, we should consider static type checking using tools like Flow or Typescript.