Next.js 15 Fullstack Web Apps · Lesson

Controlled Components and State

Manage form input values using React's controlled component pattern for predictable behavior.

Controlled Components and State is a free Next.js 15 Fullstack Web Apps lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Next.js 15 Fullstack Web Apps learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Forms: User Interaction Hub

Forms are essential for almost any web application! They allow users to input information, make choices, and interact with your app.

Think about login screens, search bars, feedback forms, or even shopping cart quantity selectors – all rely on forms.

Uncontrolled Inputs: A Quick Look

In plain HTML, input elements manage their own state. When you type into an <input>, its value updates internally.

In React, these are called uncontrolled components. While simple for static forms, they can be tricky to integrate with React's dynamic UI updates and state management.

Meet Controlled Components

Controlled components are input elements whose values are controlled by React state. Instead of the DOM managing the input's value, your React component's state becomes the 'single source of truth'.

This gives you full control over the form data, making it predictable and easier to manage.

The Core: `value` & `onChange`

Two props are key to controlled components:

value: Sets the current displayed value of the input. It should always come from your component's state.
onChange: A function that gets called whenever the input's value changes (e.g., user types). This function updates your component's state.

Controlled Text Input in Action

Let's see a basic controlled text input. We use useState to hold the input's value and update it with onChange.

Try typing in the input below:

import React, { useState } from 'react';

function App() {
  const [name, setName] = useState('');

  const handleChange = (event) => {
    setName(event.target.value);
  };

  return (
    <div>
      <label>Your Name:</label>
      <input
        type="text"
        value={name}
        onChange={handleChange}
      />
      <p>Hello, {name || 'stranger'}!</p>
    </div>
  );
}

State is the Single Source of Truth

Here's the flow for a controlled input:

The input's value prop is set by React state (e.g., name).
User types a character.
The onChange event fires, calling handleChange.
handleChange updates the state (setName).
React re-renders the component.
The input's value prop is now updated with the new state, displaying the typed character.

Other Controlled Elements

The same value and onChange pattern applies to other form elements like <textarea> and <select>.

For <select>, the value prop is set on the <select> tag itself, not on individual <option> tags.

import React, { useState } from 'react';

function App() {
  const [feedback, setFeedback] = useState('');
  const [fruit, setFruit] = useState('apple');

  return (
    <div>
      <label>Your Feedback:</label>
      <textarea
        value={feedback}
        onChange={(e) => setFeedback(e.target.value)}
        rows="3"
      />
      <p>Feedback: {feedback}</p>

      <label>Favorite Fruit:</label>
      <select
        value={fruit}
        onChange={(e) => setFruit(e.target.value)}
      >
        <option value="apple">Apple</option>
        <option value="banana">Banana</option>
        <option value="orange">Orange</option>
      </select>
      <p>Selected: {fruit}</p>
    </div>
  );
}

Why Control Matters

Controlling your form inputs provides many benefits:

Instant Validation: Validate input as the user types.
Conditional Logic: Enable/disable buttons based on input state.
Formatted Input: Automatically format phone numbers or currencies.
Easy State Access: Always know the current value of any input.
Predictable Behavior: Your UI always reflects your application's state.

Managing Many Inputs

For forms with multiple inputs, you can use a single state object and a generic onChange handler. The name attribute of the input helps identify which state property to update.

import React, { useState } from 'react';

function App() {
  const [formData, setFormData] = useState({
    username: '',
    email: ''
  });

  const handleChange = (event) => {
    const { name, value } = event.target;
    setFormData(prevData => ({ ...prevData, [name]: value }));
  };

  return (
    <form>
      <label>Username:</label>
      <input
        type="text"
        name="username"
        value={formData.username}
        onChange={handleChange}
      />
      <label>Email:</label>
      <input
        type="email"
        name="email"
        value={formData.email}
        onChange={handleChange}
      />
      <p>User: {formData.username}, Email: {formData.email}</p>
    </form>
  );
}

Controlled Input Challenge

Which of the following are key characteristics of a controlled component in React?

Recap: Controlled & Predictable

You've learned about controlled components!

They link input values directly to React state.
They use the value prop for display and an onChange handler to update state.
This pattern provides full control, enabling features like instant validation and predictable behavior.
It applies to <input>, <textarea>, and <select> elements.

Mastering controlled components is a crucial step for building robust forms in Next.js applications!

Frequently asked questions

Is the “Controlled Components and State” lesson free?

Yes — the full text of “Controlled Components and State” is free to read here on the web, and the Next.js 15 Fullstack Web Apps course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Next.js 15 Fullstack Web Apps course, upgrade to CoddyKit PRO.

What will I learn in “Controlled Components and State”?

Manage form input values using React's controlled component pattern for predictable behavior. You practise Next.js 15 Fullstack Web Apps with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Next.js 15 Fullstack Web Apps?

No prior experience is required. Next.js 15 Fullstack Web Apps on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Controlled Components and State” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Next.js 15 Fullstack Web Apps lesson?

Yes. Every Next.js 15 Fullstack Web Apps lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Controlled Components and State
Form Validation with React Hook Form
Fullstack Forms with Server Actions
File Uploads and Multipart Form Handling

← Back to Next.js 15 Fullstack Web Apps