joanrig/react-forms-online-web-ft-021119

React Controlled Components

Overview

In this lesson, we'll discuss how to set up a controlled form in React.

Objectives

1. Explain how React uses value on, e.g., <input>
2. Check whether a component is controlled or uncontrolled
3. Describe strategies for using controlled components
4. Use controlled inputs to validate values
5. Distinguish between value and defaultValue in a React controlled component

Code Along

If you want to code along there is starter code in the src folder. Make sure to run npm install && npm start to see the code in the browser.

Controlling Form Values From State

Forms in React are similar to their regular HTML counterparts. The JSX we write is almost identical. The way we store and handle form data, however, is entirely new. In React, it is often a good idea to set up controlled forms. A controlled form is a form that derives its input values from state. Consider the following:

class Form extends Component {
  state = {
    firstName: "John",
    lastName: "Henry"
  }

  render() {
    return (
      <form>
        <input type="text" name="firstName" value={this.state.firstName} />
        <input type="text" name="lastName" value={this.state.lastName} />
      </form>
    )
  }
}

With the setup above, the two text inputs will display the corresponding state values. If this.state.firstName changes to "Joe", this will be reflected in the value displayed in the first input.

This turns out to be very useful for a specific purpose - since we can set our state elsewhere, using this set up, its easy to populate forms from available data.

Imagine a user profile page with an 'Edit' button that opens a form for updating user info. When a user clicks that 'Edit' button, they expect to see a form with their user data pre-populated. This way, they can easily make small changes without rewriting all their profile info.

In a React app, all the user info displayed on a user profile would already be stored in state somewhere on the application. Its already being displayed after all. By setting the value of inputs based on state as we did above, we can bring in existing state or props and populate a form dynamically.

There is a problem, though. The set up we've created is only half finished. Right now, if we were to try and type into the inputs in the example above, they would not change and will always display "John" and "Henry".

To completely control a form, we also need our form to update state.

Updating State Via Forms

If we can change state values, React will re-render and our inputs will display the new state. Well, we know that setState is what we'll need in order to initiate a state change, but when would we fire it?

We want to fire it every time the form changes. Forms should display whatever changes a user makes, even if it is adding a single letter in an input. For this, we use an event listener React has set up for us:

<input type="text" id="firstName" onChange={event => this.handleFirstNameChange(event)} value={this.state.firstName} />
<input type="text" id="lastName" onChange={event => this.handleLastNameChange(event)} value={this.state.lastName} />

Form inputs in React come with specific events. onChange will fire every time the value of an input changes. In our example, we're invoking an anonymous function that accepts event as its argument (automatically provided by the event listener), and then calls this.handleFirstNameChange or this.handleLastNameChange, passing the event as an argument. Let's write out what these functions look like:

handleFirstNameChange = event => {
  this.setState({
    firstName: event.target.value
  })
}

handleLastNameChange = event => {
  this.setState({
    lastName: event.target.value
  })
}

The event contains data about the target, which is whatever the event was triggered on. That target, being an input, has a value attribute. This attribute is equal to whatever has been entered into input. This is not the value we provided from state. When we read event.target.value, we get whatever content is present when the event fired. In the case of our first input, that would be a combination of whatever this.state.firstName is equal to plus the last key stroke. If you pressed 's', event.target.value would equal "Johns".

In these methods, we're updating state based on event.target.value. This in turn causes a re-render... and the cycle completes. The new state values we just set are used to set the value attributes of our two inputs. From a user's perspective, the form behaves exactly how we'd expect, displaying the text that is typed. From React's perspective, we gain control over form values, giving us the ability to more easily manipulate what our inputss display, send form data to other parts of the app or out onto the internet...

Controlling forms makes it more convenient to share form values between components. Since the form values are stored in state, they are easily passed down as props, or sent upward via a function supplied in props.

More on Forms

Form elements include <input>, <textarea>, <select>, and <form> itself. When we talk about inputs in this lesson, we broadly mean the form elements (<input>, <textarea>, <select>) and not always specifically just <input>.

To control the value of these inputs, we use a prop specific to that type of input:

• For <input> and <textarea>, we use value, as we have seen.

• For <input type="checkbox"> and <input type="radio">, we use checked

• For <option>, we use selected

Each of these attributes can be set based on a state value. Each also has an onChange event listener, allowing us to update state when a user interacts with a form.

Uncontrolled vs Controlled Components

React provides us with two ways of setting and getting values in form elements. These two methods are called uncontrolled and controlled components. The differences are subtle, but it's important to recognize them — and use them accordingly (spoiler: most of the time, we'll use controlled components).

The quickest way to check if a component is controlled or uncontrolled is to check for value or defaultValue. If the component has a value prop, it is controlled (the state of the component is being controlled by React). If it doesn't have a value prop, it's an uncontrolled component. Uncontrolled components can optionally have a defaultValue prop to set its initial value. These two props (value and defaultValue) are mutually exclusive: a component is either controlled or uncontrolled, but it cannot be both.

Uncontrolled Components

In uncontrolled components, the state of the component's value is kept in the DOM itself like a regular old HTML form— in other words, the form element in question (e.g. an <input>) has its own internal state. To retrieve that value, we would need direct access to the DOM component that holds the value, or we'd have to add an onChange handler.

To set an initial value for the element, we'd use the defaultValue prop. We can't use the value prop for this: we're not using state to explicitly store its value, so the component would never update its value anymore (since we're rendering the same thing). Uncontrolled forms still work just fine in React.

To submit a form, we can use the onSubmit handler on the form element itself:

<form onSubmit={ event => this.handleSubmit(event) }>
  ...
</form>

All the form data in an uncontrolled form is accessible within the event, but accessing can sometimes be a pain, as you end up writing things like event.target.children[0].value to get the value of our first input.

handleSubmit = event => {
  event.preventDefault()
  const firstName = event.target.children[0].value
  const lastName = event.target.children[1].value
  this.sendFormDataSomewhere({ firstName, lastName })
}

On a larger form this can turn into some dense code.

Controlled component

In controlled components, we explicitly set the value of a component, and update that value in response to any changes the user makes. Just to review, lets look at some code to make things clearer:

// src/components/ControlledInput.js
import React from 'react';

class ControlledInput extends React.Component {
  state = {
    value: '',
  }

  handleChange = event => {
    this.setState({
      value: event.target.value,
    });
  }

  render() {
    return (
      <form onSubmit={event => this.handleSubmit(event)}>
        <input
          type="text"
          value={this.state.value}
          onChange={this.handleChange}
        />
      </form>
    );
  }
}

export default ControlledInput;

// src/index.js

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

import ControlledInput from './components/ControlledInput';

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

As you can see, we can easily define the initial value by setting the value property on the state to whatever we want. When you enter something into the input, the value is captured and set as the new state.

Doing something with a submitted form also ends up cleaner:

handleSubmit = event => {
  event.preventDefault()
  this.sendFormDataSomewhere(this.state)
}

In this case, our entire state object is just the controlled form data, so we can send the entire object around wherever it needs to go. Not only that, if we expanded our form to have 20 controlled inputs, this handleSubmit doesn't change. It just sends all 20 state values wherever we need them to go upon submission.

Note: Most often, submitting a form would involve sending a request to a server somewhere online. We won't get into async React just yet.

Conclusion

Using a controlled component is the preferred way to do things in React — it allows us to keep all component state in the React state, instead of relying on the DOM to retrieve the element's value through its internal state. Whenever our state changes, the component re-renders, rendering the input with the new updated value. If we don't update the state, our input wouldn't update when the user would type. In other words, we need to update our input's state programmatically.

It might seem a little counterintuitive that we need to be so verbose, but this actually opens the door to additional functionality. For example, let's say we want to write an input that only takes in a number (let's pretend there is no <input type="number">). We can now validate the data the user enters before we set it on the state, allowing us to block any invalid values. If the input is invalid, we simply avoid updating the state, preventing the input from updating. We could optionally set another state property (for example, isInvalidNumber). Using that state property, we can show an error in our component to indicate that the user tried to enter an invalid value.

If we tried to do this using an uncontrolled component, the input would be entered regardless, since we don't have control over the internal state of the input. In our onChange handler, we'd have to roll the input back to its previous value, which is pretty tedious!

Bonus - Abstracting setState When onChange is Triggered

You're still here? Well, while you are, let's talk about the onChange event we've got set up in our ControlledInput component. We have two methods in the class that seem very very similar:

handleFirstNameChange = event => {
  this.setState({
    firstName: event.target.value
  })
}

handleLastNameChange = event => {
  this.setState({
    lastName: event.target.value
  })
}

Since each one is changing a different value in our state, we've got them separated here. You can imagine that once we've got a more complicated form, this route may result in a very cluttered component. Instead of separate methods, we could actually condense this down into one abstracted component. Since event is being passed in as the argument, we have access to some of the event.target attributes that may be present.

In this example, our two inputs:

<input type="text" name="firstName" value={this.state.firstName} />
<input type="text" name="lastName" value={this.state.lastName} />

Have name attributes. If we make sure the name attributes match keys in our state, we can write a generic handleChange method like so:

handleChange = event => {
  this.setState({
    [event.target.name]: event.target.value
  })
}

If we connect this method to both of our inputs, they will both correctly update state. Why? Because for the first input, event.target.name is set to firstName, while in the second input, it is set to lastName. Each input's name attribute will change which part of state is actually updated!

Resources

• React Forms
• Controlled vs Uncontrolled - Video

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