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Learn to build a secure random password generator in JavaScript with length control, character type toggles, strength meter, copy to clipboard, and history.
What You'll Learn
- How to build a configurable character pool from selected character types
- How to generate a cryptographically stronger password with
crypto.getRandomValues() - How to implement a password strength meter based on entropy
- How to add a "Copy to clipboard" button
- How to keep a history of recently generated passwords
Project Overview
A password generator creates a random string from a configurable character pool. Users choose the length and which character types to include. A strength meter shows how secure the generated password is.
Component Structure
How It Works — Flow Diagram
text example
buildCharPool()
→ concatenate enabled character sets
→ optionally remove ambiguous chars
generatePassword(length, pool)
→ for each position: pick crypto-random index from pool
→ ensure at least one char from each enabled set
→ return shuffled result
calculateStrength(password, pool)
→ entropy = log2(poolSize^length)
→ map to Very Weak / Weak / Fair / Strong / Very StrongStep-by-Step Build
Step 1 — Character Sets
Example
const CHARSETS = {
uppercase: "ABCDEFGHIJKLMNOPQRSTUVWXYZ",
lowercase: "abcdefghijklmnopqrstuvwxyz",
digits: "0123456789",
symbols: "!@#$%^&*()_+-=[]{}|;':\",./<>?"
};
const AMBIGUOUS = /[0O1lI]/g;
Step 2 — HTML Structure
html example
<div class="pw-generator">
<h1>Password Generator</h1>
<div class="length-control">
<label>Length: <span id="lengthVal">16</span></label>
<input type="range" id="lengthSlider" min="6" max="64" value="16">
</div>
<div class="charset-options">
<label><input type="checkbox" id="inclUppercase" checked> Uppercase (A-Z)</label>
<label><input type="checkbox" id="inclLowercase" checked> Lowercase (a-z)</label>
<label><input type="checkbox" id="inclDigits" checked> Numbers (0-9)</label>
<label><input type="checkbox" id="inclSymbols" checked> Symbols (!@#...)</label>
<label><input type="checkbox" id="exclAmbiguous"> Exclude ambiguous (0, O, l, 1)</label>
</div>
<div class="output-row">
<input type="text" id="pwOutput" class="pw-output" readonly>
<button id="copyBtn" class="btn-copy">📋</button>
</div>
<div class="strength-bar-container">
<div id="strengthBar" class="strength-bar"></div>
</div>
<p id="strengthLabel" class="strength-label"></p>
<p id="entropyLabel" class="entropy-label"></p>
<button id="regenBtn" class="btn-regen">🔄 Regenerate</button>
<div class="history-section">
<h3>History</h3>
<ul id="pwHistory"></ul>
</div>
</div>Step 3 — Build Character Pool
js example
function buildPool() {
let pool = "";
if (document.getElementById("inclUppercase").checked) pool += CHARSETS.uppercase;
if (document.getElementById("inclLowercase").checked) pool += CHARSETS.lowercase;
if (document.getElementById("inclDigits").checked) pool += CHARSETS.digits;
if (document.getElementById("inclSymbols").checked) pool += CHARSETS.symbols;
if (document.getElementById("exclAmbiguous").checked) {
pool = pool.replace(AMBIGUOUS, "");
}
return pool;
}Step 4 — Generate Password (Crypto-Secure)
Example
function generatePassword(length, pool) {
if (!pool) return "";
const arr = new Uint32Array(length);
crypto.getRandomValues(arr);
// Ensure at least one char from each enabled type
const required = [];
if (document.getElementById("inclUppercase").checked) required.push(randomFromSet(CHARSETS.uppercase));
if (document.getElementById("inclLowercase").checked) required.push(randomFromSet(CHARSETS.lowercase));
if (document.getElementById("inclDigits").checked) required.push(randomFromSet(CHARSETS.digits));
if (document.getElementById("inclSymbols").checked) required.push(randomFromSet(CHARSETS.symbols));
const remaining = arr.slice(required.length).map(n => pool[n % pool.length]);
const chars = [...required, ...remaining];
// Fisher-Yates shuffle
for (let i = chars.length - 1; i > 0; i--) {
const j = Math.floor(Math.random() * (i + 1));
[chars[i], chars[j]] = [chars[j], chars[i]];
}
return chars.join("").slice(0, length);
}
function randomFromSet(set) {
const arr = new Uint32Array(1);
crypto.getRandomValues(arr);
return set[arr[0] % set.length];
}
Step 5 — Strength / Entropy Calculation
Example
function calculateEntropy(length, poolSize) {
if (!poolSize || !length) return 0;
return Math.round(length * Math.log2(poolSize));
}
function getStrengthLabel(entropy) {
if (entropy < 28) return { label: "Very Weak", class: "very-weak", pct: 20 };
if (entropy < 36) return { label: "Weak", class: "weak", pct: 40 };
if (entropy < 60) return { label: "Fair", class: "fair", pct: 60 };
if (entropy < 80) return { label: "Strong", class: "strong", pct: 80 };
return { label: "Very Strong", class: "very-strong", pct: 100 };
}
Step 6 — Render & Auto-Generate
Example
const pwHistory = [];
function render() {
const pool = buildPool();
const length = parseInt(document.getElementById("lengthSlider").value);
if (!pool) {
document.getElementById("pwOutput").value = "Select at least one character type";
return;
}
const pw = generatePassword(length, pool);
const entropy = calculateEntropy(length, pool.length);
const str = getStrengthLabel(entropy);
document.getElementById("pwOutput").value = pw;
document.getElementById("lengthVal").textContent = length;
const bar = document.getElementById("strengthBar");
bar.style.width = `${str.pct}%`;
bar.className = `strength-bar ${str.class}`;
document.getElementById("strengthLabel").textContent =
`Strength: ${str.label}`;
document.getElementById("entropyLabel").textContent =
`Entropy: ~${entropy} bits`;
pwHistory.unshift(pw);
if (pwHistory.length > 5) pwHistory.pop();
renderHistory();
}
function renderHistory() {
document.getElementById("pwHistory").innerHTML = pwHistory.map(pw => `
<li class="pw-hist-item">
<code>${pw}</code>
<button class="hist-copy-btn" data-pw="${pw}">📋</button>
</li>
`).join("");
}
Step 7 — Copy to Clipboard
Example
async function copyToClipboard(text) {
try {
await navigator.clipboard.writeText(text);
return true;
} catch {
// Fallback for HTTP contexts
const el = document.createElement("textarea");
el.value = text;
document.body.appendChild(el);
el.select();
document.execCommand("copy");
el.remove();
return true;
}
}
document.getElementById("copyBtn").addEventListener("click", async () => {
const pw = document.getElementById("pwOutput").value;
await copyToClipboard(pw);
document.getElementById("copyBtn").textContent = "✅";
setTimeout(() => { document.getElementById("copyBtn").textContent = "📋"; }, 1500);
});
Step 8 — Wire All Events & Initialize
Example
document.getElementById("regenBtn").addEventListener("click", render);
document.getElementById("lengthSlider").addEventListener("input", render);
document.querySelectorAll(".charset-options input").forEach(cb => cb.addEventListener("change", render));
document.getElementById("pwHistory").addEventListener("click", async (e) => {
const btn = e.target.closest(".hist-copy-btn");
if (btn) await copyToClipboard(btn.dataset.pw);
});
render();
Console Output
Output
buildPool() → "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$..."
pool.length = 88
generatePassword(16, pool)
→ crypto.getRandomValues([...16 random integers])
→ required chars: ["M", "k", "7", "!"]
→ remaining 12 chars from pool
→ shuffled → "P@x7!mKz#3vNqL2"
calculateEntropy(16, 88)
→ 16 × log2(88) = 16 × 6.46 = ~103 bits
getStrengthLabel(103) → { label: "Very Strong", pct: 100 }Edge Cases to Handle
| Situation | Expected Behavior |
|---|---|
| No charset selected | Warning: "Select at least one type" |
| Length shorter than required chars | Clamp length to minimum (4 if all selected) |
| Ambiguous exclusion removes all pool chars | Fallback warning |
| Copy on HTTP (no clipboard API) | Use execCommand("copy") fallback |
| Slider to max (64) | Valid long password generated |
Challenge Yourself
- Pronounceable passwords — generate phonetically readable passwords (like "korbi-9424")
- Passphrase mode — generate 4-word passphrases like "correct-horse-battery-staple"
- Password checker — paste any password and evaluate its strength
- Dark mode — style the strength bar with dark-theme colors
- Browser extension — package this as a browser extension popup
Best Practices
- Use
crypto.getRandomValues()instead ofMath.random()for security-sensitive passwords - Always guarantee at least one character from each enabled set before shuffling
- Calculate entropy as
log2(poolSize^length) = length × log2(poolSize) - Shuffle the password after placing required characters (Fisher-Yates) to avoid predictable positions
- Never log or store passwords in plaintext beyond a short session history
Continue the lesson path
Study next in this module
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