Controlled Components

Learning Goals

• Understand what "controlled components" are in React
• Implement controlled components by synchronizing input values with component state

Introduction

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

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.

Note: in the examples in this lesson, form submission functionality is omitted for simplicity.

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:

import React, { useState } from "react";

function Form() {
  const [firstName, setFirstName] = useState("John");
  const [lastName, setLastName] = useState("Henry");

  return (
    <form>
      <input type="text" value={firstName} />
      <input type="text" value={lastName} />
      <button type="submit">Submit</button>
    </form>
  );
}

export default Form;

With the setup above, the two text input elements will display the corresponding state values.

This code is not quite complete though — as it is now, there is no way to change the state. The inputs in the form above will be stuck displaying whatever state is set to.

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. We know that setFirstName and setLastName are what we'll need to initiate a state change, but when would we use them?

We want to fire it every time the input value 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, onChange, that React has set up for us:

<input type="text" onChange={handleFirstNameChange} value={firstName} />
<input type="text" onChange={handleLastNameChange} value={lastName} />

We can listen for several types of events on input fields. onChange will fire every time the value of an input changes. In our example, we're passing a callback function that accepts event as its argument. The event data being passed in is automatically provided by the onChange event listener. Let's write out what these functions look like:

function handleFirstNameChange(event) {
  setFirstName(event.target.value);
}

function handleLastNameChange(event) {
  setLastName(event.target.value);
}

The event contains data about the target, which is whatever DOM element the event was triggered on. That target, being an input element, has a value attribute. This attribute is equal to whatever is currently entered into that particular input!

Keep in mind, 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 firstName is equal to plus the last key stroke. If you pressed 's', event.target.value would equal "Johns".

Inside both functions is a function to update state. The two functions are nearly identical, with just one difference: setFirstName() changes the firstName, and setLastName() changes the lastName. The full component would look like the following:

import React, { useState } from "react";

function Form() {
  const [firstName, setFirstName] = useState("John");
  const [lastName, setLastName] = useState("Henry");

  function handleFirstNameChange(event) {
    setFirstName(event.target.value);
  }

  function handleLastNameChange(event) {
    setLastName(event.target.value);
  }

  return (
    <form>
      <input type="text" onChange={handleFirstNameChange} value={firstName} />
      <input type="text" onChange={handleLastNameChange} value={lastName} />
      <button type="submit">Submit</button>
    </form>
  );
}

export default Form;

In the handleFirstNameChange() and handleLastNameChange() functions, 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 (or restrict) what our inputss display, and 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.

Form Element Types

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>, <textarea>, and <select>, we use value, as we have seen.
• For a checkbox (<input type="checkbox">), we use checked:

import React, { useState } from "react";

function Form() {
  const [newsletter, setNewsletter] = useState(false);

  function handleNewsletterChange(event) {
    // .checked, not .value!
    setNewsletter(event.target.checked);
  }

  return (
    <form>
      <label htmlFor="newsletter">Subscribe to our Newsletter?</label>
      <input
        type="checkbox"
        id="newsletter"
        onChange={handleNewsletterChange}
        {/* checked instead of value */}
        checked={newsletter}
      />
      <button type="submit">Submit</button>
    </form>
  );
}

export default Form;

Each of the input types has an onChange event listener, allowing us to update state when a user interacts with a form. Once that happens, the value or checked attribute is then set based on the updated state value. Combining these two steps is what enables us to set up controlled forms.

Why Use Controlled Forms When We Do Not Have To

Controlled forms can be very useful for specific purposes — since we can set our state elsewhere using this setup, it's easy to populate forms from existing available data.

When we have a controlled form, the state does not need to be stored in the same component. We could store state in a parent component, instead. To demonstrate this, we'll need to create a new component. To keep it simple, we'll call this ParentComponent. ParentComponent can hold all the functions while Form just handles the display of JSX:

// src/components/ParentComponent
import React, { useState } from "react";
import Form from "./Form";

function ParentComponent() {
  const [firstName, setFirstName] = useState("John");
  const [lastName, setLastName] = useState("Henry");

  function handleFirstNameChange(event) {
    setFirstName(event.target.value);
  }

  function handleLastNameChange(event) {
    setLastName(event.target.value);
  }

  return (
    <Form
      firstName={firstName}
      lastName={lastName}
      handleFirstNameChange={handleFirstNameChange}
      handleLastNameChange={handleLastNameChange}
    />
  );
}

export default ParentComponent;

Then Form can become:

// src/components/Form
import React from "react";

function Form({
  firstName,
  lastName,
  handleFirstNameChange,
  handleLastNameChange,
}) {
  return (
    <form>
      <input type="text" onChange={handleFirstNameChange} value={firstName} />
      <input type="text" onChange={handleLastNameChange} value={lastName} />
      <button type="submit">Submit</button>
    </form>
  );
}

export default Form;

Previously, our application was rendering Form directly inside src/index.js. Now, however, we've added a component that renders Form as a child. Because of this change, you'll need to update src/index.js so that it renders ParentComponent instead of Form.

Note: If you're following along in the example files, don't forget to update index.js to point to ParentComponent. If you don't make this change, the behavior of the form inputs will appear the same, but they will just be regular HTML input fields — they will not be controlled. To verify this, you can log event.target.value from inside the handleFirstNameChange and handleLastNameChange functions in ParentComponent.

With ParentComponent, we've moved all the form logic up one level.

Being able to store controlled form data in other components opens some interesting doors for us. We could, for instance, create another component, a sibling of Form, that displays our form data as soon as a user starts filling in the form:

// src/components/DisplayData
import React from "react";

function DisplayData({ firstName, lastName }) {
  return (
    <div>
      <h1>{firstName}</h1>
      <h1>{lastName}</h1>
    </div>
  );
}

export default DisplayData;

...and adding it alongside Form (also wrapping both in a div):

// src/components/ParentComponent
import React, { useState } from "react";
import Form from "./Form";
import DisplayData from "./DisplayData";

function ParentComponent() {
  // ...
  return (
    <div>
      <Form
        firstName={firstName}
        lastName={lastName}
        handleFirstNameChange={handleFirstNameChange}
        handleLastNameChange={handleLastNameChange}
      />
      <DisplayData firstName={firstName} lastName={lastName} />
    </div>
  );
}

Now we have a component that reads from the same state we're changing with the form.

This can be a very useful way to capture user input and utilize it throughout your application, even if a server is not involved.

The opposite can also be true. 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.

Just like we did with ParentComponent, this could be achieved by populating a form with data from props! After all, if we have a React app that is displaying user information, that information is stored somewhere on the app.

Controlled Forms for Validation

It might seem a little counterintuitive that we need to be so verbose when working with forms in React, but this actually opens the door to additional functionality. For example, let's say we want to write an input that only takes the numbers 0 through 5. We can now validate the data the user enters before we set it on the state, allowing us to block any invalid values:

function Form() {
  const [number, setNumber] = useState(0);

  function handleNumberChange(event) {
    const newNumber = parseInt(event.target.value);
    if (newNumber >= 0 && newNumber <= 5) {
      setNumber(newNumber);
    }
  }

  return (
    <form>
      <input type="number" value={number} onChange={handleNumberChange} />
    </form>
  );
}

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:

function Form() {
  const [number, setNumber] = useState(0);
  const [isInvalidNumber, setIsInvalidNumber] = useState(null);

  function handleNumberChange(event) {
    const newNumber = parseInt(event.target.value);
    if (newNumber >= 0 && newNumber <= 5) {
      setNumber(newNumber);
      setIsInvalidNumber(null);
    } else {
      setIsInvalidNumber(`${newNumber} is not a valid number!`);
    }
  }

  return (
    <form>
      <input type="number" value={number} onChange={handleNumberChange} />
      {isInvalidNumber ? <span style={{ color: "red" }}>{isInvalidNumber}</span> : null}
    </form>
  );
}

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!

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.

Using a controlled form, 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 will not update when the user types. In other words, we need to update our input's state programmatically.

Resources

• React Forms