Forms and Input Handling

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Table of Content

Forms and Input Handling

Handling user input through forms is a critical aspect of web development. JavaScript provides methods to interact with form elements, validate user input, and respond to form submissions dynamically.

Accessing Form Elements

JavaScript can select form elements using document.forms, getElementById(), or querySelector().

let form = document.getElementById("myForm");
let username = document.querySelector("#username");
let email = document.forms["myForm"]["email"];

let form = document.getElementById("myForm");
let username = document.querySelector("#username");
let email = document.forms["myForm"]["email"];

let form = document.getElementById("myForm");
let username = document.querySelector("#username");
let email = document.forms["myForm"]["email"];

Getting and Setting Input Values

To get user input:

let usernameValue = document.getElementById("username").value;
console.log(usernameValue);

let usernameValue = document.getElementById("username").value;
console.log(usernameValue);

let usernameValue = document.getElementById("username").value;
console.log(usernameValue);

To set an input value dynamically:

document.getElementById("username").value = "DefaultUser";

document.getElementById("username").value = "DefaultUser";

document.getElementById("username").value = "DefaultUser";

Handling Form Submission

The submit event triggers when a form is submitted. Use event.preventDefault() to prevent page reload.

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
  console.log("Form submitted!");
});

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
  console.log("Form submitted!");
});

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
  console.log("Form submitted!");
});

Example of collecting form data:

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
    
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
  
  console.log(`Username: ${username}, Email: ${email}`);
});

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
    
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
  
  console.log(`Username: ${username}, Email: ${email}`);
});

document.getElementById("myForm").addEventListener("submit", event => {
  event.preventDefault();
    
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
  
  console.log(`Username: ${username}, Email: ${email}`);
});

Form Validation

Validating input fields ensures data correctness before submission.

Basic Validation Using JavaScript

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
    
  if (username === "" || email === "") {
    event.preventDefault();
    alert("All fields are required!");
  }
});

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
    
  if (username === "" || email === "") {
    event.preventDefault();
    alert("All fields are required!");
  }
});

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let email = document.getElementById("email").value;
    
  if (username === "" || email === "") {
    event.preventDefault();
    alert("All fields are required!");
  }
});

Validating Input Length

if (username.length < 3) {
  alert("Username must be at least 3 characters long.");
}

if (username.length < 3) {
  alert("Username must be at least 3 characters long.");
}

if (username.length < 3) {
  alert("Username must be at least 3 characters long.");
}

Validating Email Format

Use regular expressions to check if an email is valid.

let emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
if (!emailPattern.test(email)) {
  alert("Enter a valid email address.");
}

let emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
if (!emailPattern.test(email)) {
  alert("Enter a valid email address.");
}

let emailPattern = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;
if (!emailPattern.test(email)) {
  alert("Enter a valid email address.");
}

Displaying Validation Messages

Instead of alert(), error messages can be displayed near the input field.

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let errorDiv = document.getElementById("error-message");

  if (username.length < 3) {
    event.preventDefault();
    errorDiv.textContent = "Username must be at least 3 characters long.";
  } else {
    errorDiv.textContent = "";
  }
});

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let errorDiv = document.getElementById("error-message");

  if (username.length < 3) {
    event.preventDefault();
    errorDiv.textContent = "Username must be at least 3 characters long.";
  } else {
    errorDiv.textContent = "";
  }
});

document.getElementById("myForm").addEventListener("submit", event => {
  let username = document.getElementById("username").value;
  let errorDiv = document.getElementById("error-message");

  if (username.length < 3) {
    event.preventDefault();
    errorDiv.textContent = "Username must be at least 3 characters long.";
  } else {
    errorDiv.textContent = "";
  }
});

<input type="text" id="username">
<div id="error-message" style="color: red;"></div>

<input type="text" id="username">
<div id="error-message" style="color: red;"></div>

<input type="text" id="username">
<div id="error-message" style="color: red;"></div>

Handling Real-Time Input Events

JavaScript can validate inputs as users type using the input or keyup event.

document.getElementById("username").addEventListener("input", event => {
  let value = event.target.value;
  document.getElementById("live-output").textContent = value;
});

document.getElementById("username").addEventListener("input", event => {
  let value = event.target.value;
  document.getElementById("live-output").textContent = value;
});

document.getElementById("username").addEventListener("input", event => {
  let value = event.target.value;
  document.getElementById("live-output").textContent = value;
});

<input type="text" id="username">
<p>Live Output: <span id="live-output"></span></p>

<input type="text" id="username">
<p>Live Output: <span id="live-output"></span></p>

<input type="text" id="username">
<p>Live Output: <span id="live-output"></span></p>

Working with Checkboxes and Radio Buttons

Handling Checkboxes

document.getElementById("subscribe").addEventListener("change", event => {
  console.log(event.target.checked ? "Subscribed" : "Unsubscribed");
});

document.getElementById("subscribe").addEventListener("change", event => {
  console.log(event.target.checked ? "Subscribed" : "Unsubscribed");
});

document.getElementById("subscribe").addEventListener("change", event => {
  console.log(event.target.checked ? "Subscribed" : "Unsubscribed");
});

<input type="checkbox" id="subscribe">

<input type="checkbox" id="subscribe">

<input type="checkbox" id="subscribe">

Handling Radio Buttons

document.querySelectorAll('input[name="gender"]').forEach(radio => {
  radio.addEventListener("change", event => {
    console.log("Selected gender:", event.target.value);
  });
});

document.querySelectorAll('input[name="gender"]').forEach(radio => {
  radio.addEventListener("change", event => {
    console.log("Selected gender:", event.target.value);
  });
});

document.querySelectorAll('input[name="gender"]').forEach(radio => {
  radio.addEventListener("change", event => {
    console.log("Selected gender:", event.target.value);
  });
});

<input type="radio" name="gender" value="Male"> Male
<input type="radio" name="gender" value="Female">

<input type="radio" name="gender" value="Male"> Male
<input type="radio" name="gender" value="Female">

<input type="radio" name="gender" value="Male"> Male
<input type="radio" name="gender" value="Female">

Using the `select` Element

JavaScript can detect changes in dropdowns and retrieve selected values.

document.getElementById("country").addEventListener("change", event => {
  console.log("Selected country:", event.target.value);
});

document.getElementById("country").addEventListener("change", event => {
  console.log("Selected country:", event.target.value);
});

document.getElementById("country").addEventListener("change", event => {
  console.log("Selected country:", event.target.value);
});

<select id="country">
    <option value="USA">USA</option>
    <option value="UK">UK</option>
    <option value="Canada">Canada</option>
</select>

<select id="country">
    <option value="USA">USA</option>
    <option value="UK">UK</option>
    <option value="Canada">Canada</option>
</select>

<select id="country">
    <option value="USA">USA</option>
    <option value="UK">UK</option>
    <option value="Canada">Canada</option>
</select>

Auto-Filling Form Data

Using JavaScript, forms can be auto-filled dynamically.

document.getElementById("autoFill").addEventListener("click", () => {
  document.getElementById("username").value = "JohnDoe";
  document.getElementById("email").value = "john@example.com";
});

document.getElementById("autoFill").addEventListener("click", () => {
  document.getElementById("username").value = "JohnDoe";
  document.getElementById("email").value = "john@example.com";
});

document.getElementById("autoFill").addEventListener("click", () => {
  document.getElementById("username").value = "JohnDoe";
  document.getElementById("email").value = "john@example.com";
});

<button id="autoFill">Auto-Fill Form</button>

<button id="autoFill">Auto-Fill Form</button>

<button id="autoFill">Auto-Fill Form</button>

Conclusion

JavaScript allows dynamic handling of user input through forms, validation, and event listeners. The next section will focus on Web APIs, including local storage, geolocation, and Fetch API, for interacting with browser capabilities.

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