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Validate Component Input with Prop Types in React

5:11 React lesson by

React gives us a mechanism called PropTypes to ensure that the property values passed into components are of the correct type. By ensuring the correct data is passed to our components, we can avoid potential bugs and more confidently share components with a team or as open source libraries.

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React gives us a mechanism called PropTypes to ensure that the property values passed into components are of the correct type. By ensuring the correct data is passed to our components, we can avoid potential bugs and more confidently share components with a team or as open source libraries.


Just a note here.
As of React 15.5.+ PropTypes has been moved to its own package and instead of React.PropTypes
you need to do:

// After (15.5)
import React from 'react';
import PropTypes from 'prop-types';

class Component extends React.Component {
  render() {
    return <div>{this.props.text}</div>;

Component.propTypes = {
  text: PropTypes.string.isRequired,

More info here :

In reply to

Just to follow up, this shows up as a javascript error and could cause confusion and will eventually not work.

This TodoForm component represents a form that we can use to create new todo items. In order for it to function properly, it receives a prop that is used to set the value of the input. It also receives a function as a prop and that's how we pass event information, input change events in this case to its parent component.


export const TodoForm = (props) => (
        <input type = "text"

If another member of our team wanted to use this component in another part of the application we'd like to be able to ensure they are passing in the correct props. Luckily, React has a mechanism for validating our component input called propTypes. The first step in defining propTypes for a component is to reference that component and call its propTypes property.

That's going to be equal to an object. This object will have keys that match our property name. I'll start with currentTodo. We want to specify the currentTodo should be a string. We'll do that by calling React.PropTypes.string.

TodoForm.propTypes = {
    currentTodo: React.PropTypes.string

It's important to note when we're referencing PropTypes off of React to pick the type for our property we want to make sure we use the PropTypes here with a capital P.

When we're defining the propTypes object for our component we define that property with a lower case p. Now that that's done I'm going to go into the browser and open up the DevTools. I'm going to jump into App.js and I'm just going to temporarily take this.state.currentTodo out of here. I'm going to define that as a number. I'm going to save that.


<TodoForm handleInputChange={this.handleInputChange}

We'll see when the browser refreshes that the value shows up because it will turn anything into a string.


Down here we have this warning, "Failed prop type: Invalid prop 'currentTodo' of type 'number' supplied to 'TodoForm' expected 'string'" We got a very helpful message that we can go back. We can say that needs to be a string. I'll go ahead and put that value back.

<TodoForm handleInputChange={this.handleInputChange}

I'll save that and when it reloads again that warning is gone. Now I can go back to TodoForm.js and I'm going to specify that I also want a handleInputChange property. That should be of type React.PropTypes.func. I'll save that. That's going to ensure that whatever is passed into handleInputChange is a function.

TodoForm.propTypes = {
    currentTodo: React.PropTypes.string
    handleInputChange: React.PropTypes.func

I'm preventing properties from being passed that are of the wrong data type. At this point both of these properties are basically optional for this component. If you absolutely have to have those properties and in this case we do for this component to do anything want to make sure those things are passed in and not left out.

I can just come in here and I can take on an isRequired to any of the properties that I define in PropTypes. That will ensure that the property has to be passed in to avoid that warning in the console.

TodoForm.propTypes = {
    currentTodo: React.PropTypes.string.isRequired
    handleInputChange: React.PropTypes.func.isRequired

If I jump back over to the browser, open up the console. I'm just going to again temporarily update App.js. We'll say we're just not going to pass currentTodo at all.

When the browser reloads, we'll see "The prop 'currentTodo' is marked as required in 'TodoForm', but its value is 'undefined'". I can just put currentTodo back, save it. Now my warning goes away.

Let's define propTypes for a couple more components. We'll start with our TodoList component. We'll see here that this basically takes in a single prop called todos.


{ => <TodoItem key={} {...todo}/>)}

To add that propType definition I'm just going to reference Todolist.propTypes = {}. I want to specify todos is going to be of type React.PropTypes.array and I need to have that for this to work, so that .isRequired.

TodoList.propTypes = {
    todos: React.PropTypes.array.isRequired

We'll save that. When the browser reloads we don't get any errors, so all of our props are being passed in properly.

We can go to TodoItem.js. We'll drop down to the bottom and we'll say TodoItems.propTypes = {}. TodoItem receives an entire todo object, so that's going to have three properties name, which is going to be a string. We'll say that isRequired. We're also going to have isComplete. That's going to be a Boolean value, so we'll specify that with bool.

We're not going to make that isRequired because if it's undefined it will default to falsey and that's fine. We're not using it in the component yet, but we are going to need the id from the todo, so we'll also accept an id of type number. We'll say that isRequired as well.


TodoItem.propTypes = {
    name: React.PropTypes.string.isRequired,
    isComplete: React.PropTypes.bool,
    id: React.PropTypes.number.isRequired

Upon save, our browser will reload. We don't have any warnings, so everything is adhering to the propTypes we just defined.

Joel's Head
Why are we asking?