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Learn to build a palindrome checker in JavaScript with real-time detection, punctuation stripping, case normalization, and visual character highlighting.
What You'll Learn
- How to normalize a string (lowercase, strip non-alphanumeric)
- How to check a palindrome using string reversal
- How to implement an alternative two-pointer approach
- How to give visual feedback by highlighting matching characters
- How to build a history of checked words
Project Overview
A palindrome is a word or phrase that reads the same forwards and backwards (e.g., "racecar", "A man a plan a canal Panama"). This project teaches string manipulation and simple algorithm thinking.
Component Structure
How It Works — Flow Diagram
Step-by-Step Build
Step 1 — HTML Structure
html example
<div class="palindrome-app">
<h1>Palindrome Checker</h1>
<div class="input-row">
<input type="text" id="wordInput" placeholder="Enter a word or phrase..." autocomplete="off">
<button id="checkBtn" class="btn-check">Check</button>
</div>
<div id="result" class="result hidden"></div>
<div id="visualizer" class="visualizer hidden"></div>
<div class="history-section">
<h3>History</h3>
<ul id="historyList" class="history-list"></ul>
<button id="clearHistory" class="btn-clear">Clear History</button>
</div>
</div>Step 2 — Core Logic
Example
function normalize(str) {
return str.toLowerCase().replace(/[^a-z0-9]/g, "");
}
function isPalindrome(str) {
const clean = normalize(str);
const reversed = clean.split("").reverse().join("");
return { clean, reversed, result: clean === reversed };
}
Step 3 — Two-Pointer Alternative (Educational)
Example
function isPalindromeTwoPointer(str) {
const s = normalize(str);
let left = 0, right = s.length - 1;
while (left < right) {
if (s[left] !== s[right]) return false;
left++;
right--;
}
return true;
}
// Both approaches give the same result — show this to learners
Step 4 — Visualizer (Character Matching)
Example
function buildVisualizer(clean) {
const mid = Math.floor(clean.length / 2);
const html = clean.split("").map((char, i) => {
const mirrorIdx = clean.length - 1 - i;
const matches = char === clean[mirrorIdx];
return `<span class="char ${matches ? "match" : "mismatch"}">${char.toUpperCase()}</span>`;
}).join("");
document.getElementById("visualizer").innerHTML = html;
document.getElementById("visualizer").classList.remove("hidden");
}
Step 5 — Show Result
js example
function check() {
const input = document.getElementById("wordInput").value.trim();
if (!input) return;
const { clean, result } = isPalindrome(input);
if (!clean) {
document.getElementById("result").textContent = "⚠️ No alphanumeric characters found";
document.getElementById("result").className = "result warning";
document.getElementById("result").classList.remove("hidden");
return;
}
const resultEl = document.getElementById("result");
resultEl.classList.remove("hidden");
if (result) {
resultEl.textContent = `✅ "${input}" is a palindrome!`;
resultEl.className = "result success";
} else {
resultEl.textContent = `❌ "${input}" is NOT a palindrome`;
resultEl.className = "result failure";
}
buildVisualizer(clean);
addToHistory(input, result);
}Step 6 — History
Example
const history = [];
function addToHistory(word, isPalin) {
history.unshift({ word, isPalin, timestamp: Date.now() });
renderHistory();
}
function renderHistory() {
const ul = document.getElementById("historyList");
ul.innerHTML = history.length === 0
? `<li class="empty">No checks yet</li>`
: history.map(h => `
<li class="history-item ${h.isPalin ? "pass" : "fail"}">
<span class="history-icon">${h.isPalin ? "✅" : "❌"}</span>
<span class="history-word">${h.word}</span>
</li>
`).join("");
}
document.getElementById("clearHistory").addEventListener("click", () => {
history.length = 0;
renderHistory();
});
Step 7 — Wire Events
Example
document.getElementById("checkBtn").addEventListener("click", check);
document.getElementById("wordInput").addEventListener("keydown", (e) => {
if (e.key === "Enter") check();
});
// Real-time indicator while typing
document.getElementById("wordInput").addEventListener("input", (e) => {
const { clean, result } = isPalindrome(e.target.value);
const resultEl = document.getElementById("result");
if (!clean) { resultEl.classList.add("hidden"); return; }
resultEl.classList.remove("hidden");
resultEl.textContent = result ? "✅ Palindrome!" : "❌ Not a palindrome";
resultEl.className = `result ${result ? "success" : "failure"}`;
});
Console Output
Output
normalize("racecar") → "racecar"
isPalindrome("racecar")
→ clean = "racecar"
→ reversed = "racecar"
→ true ✅
normalize("A man, a plan, a canal: Panama")
→ "amanaplanacanalpanama"
→ reversed = "amanaplanacanalpanama"
→ true ✅
isPalindrome("hello")
→ clean = "hello"
→ reversed = "olleh"
→ false ❌
isPalindromeTwoPointer("racecar")
→ [r,a,c,e,c,a,r]
→ r===r ✓, a===a ✓, c===c ✓, mid reached → true ✅Edge Cases to Handle
| Input | Expected |
|---|---|
"A man a plan a canal Panama" | ✅ (after normalize) |
"Was it a car or a cat I saw" | ✅ |
"Race a car" | ❌ |
" " | Warning: no alphanumeric chars |
"a" | ✅ (single char is always a palindrome) |
"" | Nothing happens (empty guard) |
Challenge Yourself
- Number palindrome — check if a number reads the same in binary
- Sentence highlighting — mark ignored characters (spaces, punctuation) in grey
- Longest palindrome in a string — find the longest palindromic substring
- Language selection — support non-Latin alphabets with Unicode normalization
- Animated check — animate a visual scan showing character pairs being matched
Best Practices
- Always normalize first —
toLowerCase()and strip punctuation - Write
isPalindrome()as a pure function that takes and returns values (no DOM access) - The
split("").reverse().join("")pattern is idiomatic JavaScript - For large strings, prefer the two-pointer approach — O(n) space, same O(n) time
- Real-time
inputfeedback is great UX but keep the display lightweight to avoid jank
Continue the lesson path
Study next in this module
01Mini ProjectsBuild a JavaScript OTP Input Field - Step by Step Project TutorialLearn to build an accessible OTP/PIN input field in JavaScript with auto-focus, backspace navigation, paste support, and timer-based resend functionality.02Mini ProjectsBuild a JavaScript Price Range Slider - Step by Step Project TutorialLearn to build a dual-handle price range slider in JavaScript with min/max thumb dragging, range highlighting, live price display, and product filtering.03Mini ProjectsBuild a JavaScript Notes App - Step by Step Project TutorialLearn to build a notes app in JavaScript with create, edit, delete, search, pin, and localStorage persistence. Covers markdown preview and color coding.04Mini ProjectsBuild a JavaScript Prime Number Checker - Step by Step Project TutorialLearn to build a prime number checker in JavaScript with trial division, Sieve of Eratosthenes, prime factorization, and range prime finder visualization.