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Learn to build an email validator in JavaScript using regex, real-time feedback, common domain suggestions, and bulk validation. A great regex and DOM project.
What You'll Learn
- How to write and understand an email validation regex
- How to give real-time feedback as the user types
- How to break the regex into parts so you know what each part does
- How to suggest common domain corrections (gmial.com → gmail.com)
- How to validate a list of emails in bulk
Project Overview
An email validator checks whether a string follows the standard email format user@domain.tld. It provides instant visual feedback and can also catch common typos in domain names.
Component Structure
How It Works — Flow Diagram
Step-by-Step Build
Step 1 — HTML Structure
html example
<div class="validator-container">
<h1>Email Validator</h1>
<div class="input-group">
<input type="text" id="emailInput" placeholder="Enter email address..." autocomplete="off">
<button id="checkBtn" class="btn-check">Check</button>
</div>
<div id="feedback" class="feedback"></div>
<div id="suggestion" class="suggestion"></div>
<hr>
<h2>Bulk Validator</h2>
<textarea id="bulkInput" rows="6" placeholder="Paste emails here, one per line..."></textarea>
<button id="bulkBtn" class="btn-bulk">Validate All</button>
<ul id="bulkResults" class="bulk-results"></ul>
</div>Step 2 — The Email Regex (Explained)
Example
// Breakdown of the regex:
// [a-zA-Z0-9._%+\-]+ → local part: letters, digits, dots, %, +, -
// @ → literal at sign
// [a-zA-Z0-9.\-]+ → domain name
// \. → literal dot
// [a-zA-Z]{2,} → TLD: at least 2 letters (com, io, dev, co.uk...)
const EMAIL_REGEX = /^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$/;
function validateEmail(email) {
return EMAIL_REGEX.test(email.trim().toLowerCase());
}
Step 3 — Real-Time Feedback
Example
const emailInput = document.getElementById("emailInput");
const feedback = document.getElementById("feedback");
const suggestion = document.getElementById("suggestion");
emailInput.addEventListener("input", () => {
const value = emailInput.value.trim();
suggestion.textContent = "";
if (!value) {
feedback.textContent = "";
feedback.className = "feedback";
return;
}
if (validateEmail(value)) {
feedback.textContent = "✅ Valid email address";
feedback.className = "feedback valid";
} else {
feedback.textContent = `❌ ${getErrorReason(value)}`;
feedback.className = "feedback invalid";
const fix = suggestDomain(value);
if (fix) suggestion.textContent = `💡 Did you mean: ${fix}?`;
}
});
Step 4 — Error Reason Helper
Example
function getErrorReason(email) {
if (!email.includes("@")) return "Missing @ symbol";
const [local, domain] = email.split("@");
if (!local) return "Missing username before @";
if (!domain) return "Missing domain after @";
if (!domain.includes(".")) return "Missing dot in domain";
const tld = domain.split(".").pop();
if (tld.length < 2) return "Domain extension too short";
return "Invalid email format";
}
Step 5 — Domain Typo Suggestions
Example
const COMMON_DOMAINS = ["gmail.com", "yahoo.com", "outlook.com", "hotmail.com", "icloud.com"];
function suggestDomain(email) {
if (!email.includes("@")) return null;
const typed = email.split("@")[1];
if (!typed) return null;
// simple Levenshtein-based closest match
let best = null, bestDist = Infinity;
COMMON_DOMAINS.forEach(d => {
const dist = levenshtein(typed, d);
if (dist < bestDist && dist <= 3) { bestDist = dist; best = d; }
});
return best ? email.split("@")[0] + "@" + best : null;
}
function levenshtein(a, b) {
const dp = Array.from({ length: a.length + 1 }, (_, i) => [i]);
for (let j = 0; j <= b.length; j++) dp[0][j] = j;
for (let i = 1; i <= a.length; i++)
for (let j = 1; j <= b.length; j++)
dp[i][j] = a[i-1] === b[j-1]
? dp[i-1][j-1]
: 1 + Math.min(dp[i-1][j], dp[i][j-1], dp[i-1][j-1]);
return dp[a.length][b.length];
}
Step 6 — Bulk Validator
Example
document.getElementById("bulkBtn").addEventListener("click", () => {
const lines = document.getElementById("bulkInput").value
.split("\n")
.map(l => l.trim())
.filter(Boolean);
const ul = document.getElementById("bulkResults");
ul.innerHTML = "";
lines.forEach(email => {
const li = document.createElement("li");
const ok = validateEmail(email);
li.className = ok ? "valid" : "invalid";
li.textContent = `${ok ? "✅" : "❌"} ${email}`;
ul.appendChild(li);
});
});
Console Output
Output
validateEmail("john@gmail.com") → true ✅
validateEmail("john@gmial.com") → true ✅ (but suggests gmail.com)
validateEmail("johngmail.com") → false ❌ Missing @ symbol
validateEmail("john@") → false ❌ Missing domain after @
validateEmail("john@g.c") → false ❌ Domain extension too short
validateEmail("john+filter@sub.io")→ true ✅ (+ and sub-domains supported)Edge Cases to Handle
| Input | Expected Result |
|---|---|
" john@gmail.com " | trim() clears whitespace → valid |
john@gmail.com. | Trailing dot → invalid |
a@b.co | Short but valid TLD → valid |
"@gmail.com" | Missing username → invalid |
john@@gmail.com | Double @ → invalid |
Challenge Yourself
- MX record check — use a public DNS API to verify the domain actually has mail records
- Disposable email detection — block known temp-mail domains
- CSV import — let users upload a
.csvfile and validate the email column - Copy valid emails — add a "Copy Valid" button to clipboard
- Visual stats — show a donut chart: % valid vs invalid from bulk validation
Best Practices
- Use
trim()andtoLowerCase()before every validation check - Don't over-restrict the local part —
+,.,%are valid in real email addresses - The regex is good enough for UI validation; for critical cases use a backend check
- Show specific error reasons rather than a generic "invalid email" message
- Levenshtein distance is great for small domain suggestion lists
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Build a JavaScript Email Validator - Step by Step Project Tutorial.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
Search Terms
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