Client-Side Validation
Validate input before sending it anywhere.
Client-Side Validation is a free JavaScript Academy lesson on CoddyKit — lesson 3 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 JavaScript Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Validate on the Client
Client-side validation gives instant feedback and a smoother experience. Remember it is a convenience, not security: always validate on the server too.
Validating on Submit
A common pattern checks fields when the form is submitted, canceling submission if anything is invalid.
form.addEventListener("submit", (e) => {
if (!isValid()) {
e.preventDefault();
}
});Checking a Required Field
Trim the value and reject empty input. Trimming avoids accepting strings of only spaces.
function checkName() {
const v = nameInput.value.trim();
return v.length > 0;
}Length Rules
Compare the trimmed length against minimum and maximum bounds for fields like passwords.
function checkPassword() {
const v = passwordInput.value;
return v.length >= 8 && v.length <= 64;
}Pattern Matching
Regular expressions test formats such as email shapes. Keep patterns simple and pair them with server checks.
function checkEmail() {
const v = emailInput.value.trim();
return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(v);
}Showing Error Messages
Display feedback near the field using a dedicated error element and textContent to stay safe from XSS.
function showError(field, msg) {
const err = field.nextElementSibling;
err.textContent = msg;
}Clearing Errors
Reset the message when the field becomes valid so old errors do not linger.
function clearError(field) {
field.nextElementSibling.textContent = "";
}Live Validation
Validate on the input or blur event for real-time feedback as the user types or leaves a field.
emailInput.addEventListener("blur", () => {
if (!checkEmail()) showError(emailInput, "Invalid email");
else clearError(emailInput);
});Accessibility with aria-invalid
Set aria-invalid and link errors with aria-describedby so assistive technology announces problems.
emailInput.setAttribute("aria-invalid", "true");Focusing the First Error
On a failed submit, move focus to the first invalid field so users can fix it immediately.
function focusFirstError(fields) {
const bad = fields.find((f) => !f.valid);
if (bad) bad.el.focus();
}Validate Server-Side Too
Client validation can be bypassed by disabling JavaScript or crafting requests. Treat the server as the source of truth.
Quick Check
Security mindset.
Recap
Manual validation checks fields (required, length, pattern) and cancels submit on failure with preventDefault. Show and clear messages safely with textContent, validate live on input/blur, improve accessibility with aria-invalid, and focus the first error. Always re-validate on the server.
Frequently asked questions
Is the “Client-Side Validation” lesson free?
Yes — the full text of “Client-Side Validation” is free to read here on the web, and the JavaScript Academy 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 JavaScript Academy course, upgrade to CoddyKit PRO.
What will I learn in “Client-Side Validation”?
Validate input before sending it anywhere. You practise JavaScript Academy 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 JavaScript Academy?
No prior experience is required. JavaScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Client-Side Validation” 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 JavaScript Academy lesson?
Yes. Every JavaScript Academy 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
- Reading Input Values
- The submit Event
- Client-Side Validation
- The Constraint Validation API