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Forms and Input Handling

Last Updated: Apr 13, 2025

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"];

Getting and Setting Input Values

To get user input:

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

To set an input value dynamically:

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!");
});

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}`);
});

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!");
  }
});

Validating Input Length

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.");
}

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 = "";
  }
});
<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;
});
<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");
});
<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);
  });
});
<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);
});
<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";
});
<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.

Previous

compatibility-and-performance

Next

threading-model

Elements

Forms and Input Handling

Last Updated: Apr 13, 2025

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"];

Getting and Setting Input Values

To get user input:

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

To set an input value dynamically:

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!");
});

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}`);
});

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!");
  }
});

Validating Input Length

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.");
}

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 = "";
  }
});
<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;
});
<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");
});
<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);
  });
});
<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);
});
<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";
});
<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.

Previous

compatibility-and-performance

Next

threading-model

Forms and Input Handling

Last Updated: Apr 13, 2025

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"];

Getting and Setting Input Values

To get user input:

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

To set an input value dynamically:

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!");
});

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}`);
});

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!");
  }
});

Validating Input Length

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.");
}

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 = "";
  }
});
<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;
});
<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");
});
<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);
  });
});
<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);
});
<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";
});
<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.

Previous

compatibility-and-performance

Next

threading-model
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Research Reports

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Contact Us

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Legal Hub

User Privacy Statement

Security Posture

Terms & Conditions

FOLLOW US

X (Twitter)

LinkedIn

Copyright ©2025. Open Ecosystems

ECOSYSTEMS

Teladoc + Lyft

See All

RESOURCES

Case Studies

Books

Research Reports

Blogs

ORGANIZATION

Services

Careers

Contact Us

Press

PARTNERS

Ecosystem Partners

People

LEGAL

Legal Hub

User Privacy Statement

Security Posture

Terms & Conditions

FOLLOW US

X (Twitter)

LinkedIn

Copyright ©2025. Open Ecosystems