Forms in React

Controlled components

In vanilla JavaScript, form inputs manage their own values. In React, you’ll often manage input values with state. This is called a controlled component — React state is the single source of truth.

import { useState } from 'react';

function SearchBar() {
  const [query, setQuery] = useState("");

  return (
    <input
      type="text"
      value={query}
      onChange={e => setQuery(e.target.value)}
      placeholder="Search..."
    />
  );
}

Two things make an input “controlled”:

value — set from state (React controls what’s displayed)
onChange — updates state when the user types

Without both, the input either won’t display the right value or won’t respond to typing.

If you set value without onChange, the input becomes read-only — the user can’t type in it. React will warn you about this in the console. Always pair value with onChange.

All input types

The value + onChange pattern works the same way for most common input types:

function AllInputTypes() {
  const [name, setName] = useState("");
  const [age, setAge] = useState("");
  const [bio, setBio] = useState("");
  const [agreed, setAgreed] = useState(false);
  const [role, setRole] = useState("viewer");
  const [plan, setPlan] = useState("free");

  return (
    <form>
      {/* Text input */}
      <input
        type="text"
        value={name}
        onChange={e => setName(e.target.value)}
      />

      {/* Number input */}
      <input
        type="number"
        value={age}
        onChange={e => setAge(e.target.value)}
      />

      {/* Textarea */}
      <textarea
        value={bio}
        onChange={e => setBio(e.target.value)}
      />

      {/* Checkbox — uses checked, not value */}
      <label>
        <input
          type="checkbox"
          checked={agreed}
          onChange={e => setAgreed(e.target.checked)}
        />
        I agree to the terms
      </label>

      {/* Select dropdown */}
      <select value={role} onChange={e => setRole(e.target.value)}>
        <option value="viewer">Viewer</option>
        <option value="editor">Editor</option>
        <option value="admin">Admin</option>
      </select>

      {/* Radio buttons — same state, different values */}
      <label>
        <input
          type="radio"
          name="plan"
          value="free"
          checked={plan === "free"}
          onChange={e => setPlan(e.target.value)}
        />
        Free
      </label>
      <label>
        <input
          type="radio"
          name="plan"
          value="pro"
          checked={plan === "pro"}
          onChange={e => setPlan(e.target.value)}
        />
        Pro
      </label>
    </form>
  );
}

Checkboxes use checked and e.target.checked. Most other inputs use value and e.target.value. (File inputs are a special case and are usually handled as uncontrolled inputs.)

Even for <input type="number">, e.target.value is still a string. Convert it with Number(...) (or parseInt/parseFloat) when you actually need a number.

Handling form submission

Use onSubmit on the <form> element, not onClick on the button:

function CreateUserForm() {
  const [name, setName] = useState("");
  const [email, setEmail] = useState("");
  const [submitting, setSubmitting] = useState(false);

  async function handleSubmit(e) {
    e.preventDefault(); // Stop the page from reloading

    setSubmitting(true);
    try {
      const response = await fetch("/api/users", {
        method: "POST",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify({ name, email }),
      });

      if (!response.ok) throw new Error(`HTTP ${response.status}`);

      const newUser = await response.json();
      console.log("Created:", newUser);

      // Clear the form
      setName("");
      setEmail("");
    } catch (err) {
      console.error("Failed to create user:", err);
    } finally {
      setSubmitting(false);
    }
  }

  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        value={name}
        onChange={e => setName(e.target.value)}
        placeholder="Name"
        required
      />
      <input
        type="email"
        value={email}
        onChange={e => setEmail(e.target.value)}
        placeholder="Email"
        required
      />
      <button type="submit" disabled={submitting}>
        {submitting ? "Creating..." : "Create User"}
      </button>
    </form>
  );
}

Always use e.preventDefault() in your submit handler. Without it, the browser reloads the page (the default HTML form behavior), which destroys your React app state. Use onSubmit on the form, not onClick on the button — this way pressing Enter also submits.

Managing multiple fields with one state object

When forms have many fields, use a single state object instead of separate useState calls:

function RegistrationForm() {
  const [formData, setFormData] = useState({
    name: "",
    email: "",
    password: "",
    role: "viewer",
  });

  function handleChange(e) {
    const { name, value } = e.target;
    setFormData(prev => ({
      ...prev,
      [name]: value,
    }));
  }

  function handleSubmit(e) {
    e.preventDefault();
    console.log("Submitting:", formData);
    // POST to API...
  }

  return (
    <form onSubmit={handleSubmit}>
      <input
        name="name"
        type="text"
        value={formData.name}
        onChange={handleChange}
        placeholder="Name"
      />
      <input
        name="email"
        type="email"
        value={formData.email}
        onChange={handleChange}
        placeholder="Email"
      />
      <input
        name="password"
        type="password"
        value={formData.password}
        onChange={handleChange}
        placeholder="Password"
      />
      <select name="role" value={formData.role} onChange={handleChange}>
        <option value="viewer">Viewer</option>
        <option value="editor">Editor</option>
        <option value="admin">Admin</option>
      </select>
      <button type="submit">Register</button>
    </form>
  );
}

The key trick is [name]: value — a computed property name. The input’s name attribute matches the key in state, so one handleChange function works for all fields.

Every input needs a name attribute that matches the state key. handleChange reads e.target.name to know which field to update. This is the standard pattern for multi-field forms in React.

Basic validation

Show errors per field and validate on submit:

function LoginForm() {
  const [formData, setFormData] = useState({ email: "", password: "" });
  const [errors, setErrors] = useState({});

  function handleChange(e) {
    const { name, value } = e.target;
    setFormData(prev => ({ ...prev, [name]: value }));

    // Clear error when user starts typing
    if (errors[name]) {
      setErrors(prev => ({ ...prev, [name]: null }));
    }
  }

  function validate() {
    const newErrors = {};

    if (!formData.email) {
      newErrors.email = "Email is required";
    } else if (!formData.email.includes("@")) {
      newErrors.email = "Invalid email address";
    }

    if (!formData.password) {
      newErrors.password = "Password is required";
    } else if (formData.password.length < 8) {
      newErrors.password = "Password must be at least 8 characters";
    }

    return newErrors;
  }

  function handleSubmit(e) {
    e.preventDefault();

    const newErrors = validate();
    if (Object.keys(newErrors).length > 0) {
      setErrors(newErrors);
      return; // Don't submit
    }

    console.log("Valid! Submitting:", formData);
    // POST to API...
  }

  return (
    <form onSubmit={handleSubmit}>
      <div>
        <input
          name="email"
          type="email"
          value={formData.email}
          onChange={handleChange}
          placeholder="Email"
        />
        {errors.email && <p className="error">{errors.email}</p>}
      </div>

      <div>
        <input
          name="password"
          type="password"
          value={formData.password}
          onChange={handleChange}
          placeholder="Password"
        />
        {errors.password && <p className="error">{errors.password}</p>}
      </div>

      <button type="submit">Log In</button>
    </form>
  );
}

The pattern: validate on submit, show errors with &&, clear errors when the user starts fixing them.

For production apps, consider a form library like React Hook Form. But this manual pattern teaches you exactly what’s happening and works well for simple forms. Understand this pattern first, then add libraries when you need them.

Edit forms — pre-populated from data

Edit forms start with existing data passed via props:

function EditUserForm({ user, onSave }) {
  const [formData, setFormData] = useState({
    name: user.name,
    email: user.email,
    bio: user.bio || "",
  });

  function handleChange(e) {
    const { name, value } = e.target;
    setFormData(prev => ({ ...prev, [name]: value }));
  }

  async function handleSubmit(e) {
    e.preventDefault();

    const response = await fetch(`/api/users/${user.id}`, {
      method: "PUT",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify(formData),
    });

    if (response.ok) {
      const updated = await response.json();
      onSave(updated); // Notify parent
    }
  }

  return (
    <form onSubmit={handleSubmit}>
      <input name="name" value={formData.name} onChange={handleChange} />
      <input name="email" value={formData.email} onChange={handleChange} />
      <textarea name="bio" value={formData.bio} onChange={handleChange} />
      <button type="submit">Save Changes</button>
    </form>
  );
}

Initialize state from props for edit forms. The onSave callback lets the parent component know the data was updated — this is the child-to-parent communication pattern you’ll learn in the next lesson.

What’s next?

You can build forms and handle user input. The last piece: how to structure your components together — which components own the data, which ones just display it, and how they communicate.

Component composition

Structure your React app with smart and presentational components

Getting started with React

Core concepts

React patterns

Controlled components

All input types

Handling form submission

Managing multiple fields with one state object

Basic validation

Edit forms — pre-populated from data

What’s next?

Component composition

Getting started with React

Core concepts

React patterns

​Controlled components

​All input types

​Handling form submission

​Managing multiple fields with one state object

​Basic validation

​Edit forms — pre-populated from data

​What’s next?

Component composition

Controlled components

All input types

Handling form submission

Managing multiple fields with one state object

Basic validation

Edit forms — pre-populated from data

What’s next?