HTML Editor: Online HTML Editor with Real-Time Preview (version 6)

Question

Overview

HTML Editor is a web-based HTML editor designed for web developers, designers, and learners. It offers a lightweight, minimalist environment for writing and previewing HTML, CSS, and JavaScript code in real time. The editor also supports creating, opening, and editing various text-based file types, such as .txt, .css, .js, .svg, and more.

This project is hosted on GitLab and licensed under the MIT License.

Changelog

• Find and replace
  Added a fully featured replace bar with:
  • Find and replace inputs
  • Match case, whole word, and regex modes
  • Replace All button with replacement count feedback
  • Undo button for last replacement
• Document metrics
  Live character and byte counts in the footer, formatted with thousand-separator (Intl.NumberFormat)
• Debounced updates
  Debounced preview rendering and document metrics updates for smoother performance
• Refactored JavaScript
  Modular helper functions (debounce, toggleClass, interpretEscapeSequences, separateThousands, and others) improve readability and reuse
• UI and CSS enhancements
  • Grouped header controls into <div> wrappers
  • Introduced utility classes (.toggle, .active, .number)
  • Expanded font-size options (up to 24px)
  • Consistent sizing using rem units
• Extended state persistence
  localStorage now saves and restores replace bar state, find and replace inputs, and document metrics, in addition to existing settings

Specific review requests

• Best practices: How well does my code adhere to best practices and coding conventions?
• Code structure and readability: Are there any improvements I can make to the organization and clarity of my code?
• Performance: Are there any optimizations that can enhance the performance of the editor?
• Features and usability: Do you have any suggestions for additional features or improvements to the existing ones?
• Accessibility and semantics: Is the markup appropriate?
• Edge cases: Can you spot any scenarios where the replace or preview operation might fail?

Source code

CSS

/* ==================== */
/* Base and Component Styles */
/* ==================== */
html,
body {
 height: 100%;
 padding: 0;
 margin: 0;
}
body {
 display: flex;
 flex-direction: column;
}
header {
 background: linear-gradient(#FFF, #CCC);
}
footer {
 background: linear-gradient(#CCC, #FFF);
 padding: 5px;
}
header div,
footer {
 display: flex;
 flex-wrap: wrap;
 align-items: center;
 gap: 5px;
}
header div {
 margin: 5px;
}
main {
 display: flex;
 flex: 1;
}
main.vertical {
 flex-direction: column;
}
header *,
footer * {
 font: 0.75rem Arial, sans-serif;
 color: #333;
}
select,
button,
input {
 margin: 0;
}
label[for="editorSizeInput"],
#share {
 margin-left: auto;
}
#editorSizeInput,
iframe {
 padding: 0;
}
#footerContainerToggle {
 width: 1rem;
 height: 1rem;
 padding: 0;
 border-bottom-width: 0.3125rem;
 border-radius: 0;
}
#copyButton {
 padding: 0;
 border: 0;
 background: transparent;
 cursor: pointer;
}
img {
 display: block;
 width: 1rem;
 height: 1rem;
}
main div {
 position: relative;
}
#previewerWrapper {
 border-left: 5px solid #CCC;
}
main.vertical #previewerWrapper {
 border-left: 0;
 border-top: 5px solid #CCC;
}
main div * {
 position: absolute;
 width: 100%;
 height: 100%;
 border: 0;
 margin: 0;
 background: #FFF;
}
textarea {
 box-sizing: border-box;
 padding: 5px;
 outline: 0;
 resize: none;
 color: #333;
}
textarea.dark {
 background: #333;
 color: #FFF;
}
/* ==================== */
/* Utilities */
/* ==================== */
.toggle {
 padding: 2px 6px;
 border: 1px solid #666;
 border-radius: 12px;
 background: transparent;
}
.active {
 border-color: #333;
 background: #FFF;
}
.number {
 font-family: monospace;
}
.hidden {
 display: none;
}

HTML

<header>
 <div>
 <a href="" download="template.html" id="downloadLink" title="Download HTML document">Download</a>
 <label for="fontSizeSelector">Font size</label>
 <select id="fontSizeSelector">
 <option>12</option>
 <option>13</option>
 <option selected>14</option>
 <option>15</option>
 <option>16</option>
 <option>17</option>
 <option>18</option>
 <option>20</option>
 <option>22</option>
 <option>24</option>
 </select>
 <label for="previewSelector">Preview</label>
 <select id="previewSelector">
 <option>Instant</option>
 <optgroup label="Delayed">
 <option value="500" selected>0.5 s</option>
 <option value="1000">1.0 s</option>
 <option value="1500">1.5 s</option>
 <option value="2000">2.0 s</option>
 </optgroup>
 <option>Manual</option>
 </select>
 <button type="button" id="runButton">Run</button>
 <button type="button" id="resetButton">Reset</button>
 <button type="button" id="selectButton">Select</button>
 <input type="file" accept="text/html" id="fileInput">
 <label for="editorSizeInput">Editor size</label>
 <input type="range" id="editorSizeInput">
 <output for="editorSizeInput" class="number" id="editorSizeOutput"></output>
 <button type="button" class="toggle" id="verticalViewToggle">Vertical View</button>
 <button type="button" class="toggle" id="darkEditorToggle">Dark Editor</button>
 <button type="button" class="toggle" id="spellcheckToggle">Spellcheck</button>
 <button type="button" class="toggle" id="replaceBarToggle">Replace</button>
 <button type="button" class="toggle" id="footerContainerToggle" title="Toggle footer"></button>
 </div>
 <div id="replaceBar">
 <label for="findInput">Find</label>
 <input type="text" id="findInput">
 <label for="replaceInput">Replace</label>
 <input type="text" id="replaceInput">
 <button type="button" class="toggle" id="matchCaseToggle">Match Case</button>
 <button type="button" class="toggle" id="wholeWordToggle">Whole Word</button>
 <button type="button" class="toggle" id="regexModeToggle">Regex</button>
 <button type="button" id="replaceAllButton">Replace All</button>
 <button type="button" id="undoButton" disabled>Undo</button>
 <output for="findInput matchCaseToggle wholeWordToggle regexModeToggle editor" id="replaceFeedback"></output>
 </div>
</header>
<main>
 <div id="editorWrapper">
 <textarea><!doctype html>
<html lang="en">
<head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>HTML Document Template</title>
 <style>
 p {
 font-family: Arial, sans-serif;
 }
 </style>
</head>
<body>
 <p>Hello, world!</p>
 <script>
 console.log(document.querySelector('p').textContent);
 </script>
</body>
</html></textarea>
 </div>
 <div id="previewerWrapper">
 <iframe></iframe>
 </div>
</main>
<footer>
 <output for="editor">
 <span class="number" id="characterCount"></span> <span id="characterLabel"></span> | <span class="number" id="byteCount"></span> <span id="byteLabel"></span>
 </output>
 <span id="share">Share</span>
 <a href="https://x.com/intent/post?text=HTML%20Editor%3A%20Online%20HTML%20Editor%20with%20Real-Time%20Preview&url=https%3A%2F%2Fhtmleditor.gitlab.io" target="_blank"><img src="images/x.svg" alt="X"></a>
 <a href="https://www.facebook.com/sharer.php?u=https%3A%2F%2Fhtmleditor.gitlab.io" target="_blank"><img src="images/facebook.svg" alt="Facebook"></a>
 <a href="https://www.linkedin.com/feed/?shareActive=true&shareUrl=https%3A%2F%2Fhtmleditor.gitlab.io" target="_blank"><img src="images/linkedin.svg" alt="LinkedIn"></a>
 <a href="mailto:?subject=HTML%20Editor%3A%20Online%20HTML%20Editor%20with%20Real-Time%20Preview&body=https%3A%2F%2Fhtmleditor.gitlab.io" target="_blank"><img src="images/email.svg" alt="Email"></a>
 <button type="button" id="copyButton"><img src="images/link.svg" alt="Link"></button>
 <output id="copyNotification"></output>
 <address><a href="https://codereview.stackexchange.com/questions/296011/html-editor-online-html-editor-with-real-time-preview-version-6" title="Code Review Stack Exchange">Feedback</a> | Created by <a href="https://mori.pages.dev" rel="author">Mori</a></address>
</footer>

JavaScript

// ====================
// Constants and Variables
// ====================
const downloadLink = document.getElementById('downloadLink');
const fontSizeSelector = document.getElementById('fontSizeSelector');
const previewSelector = document.getElementById('previewSelector');
const runButton = document.getElementById('runButton');
const resetButton = document.getElementById('resetButton');
const selectButton = document.getElementById('selectButton');
const fileInput = document.getElementById('fileInput');
const editorSizeInput = document.getElementById('editorSizeInput');
const editorSizeOutput = document.getElementById('editorSizeOutput');
const verticalViewToggle = document.getElementById('verticalViewToggle');
const darkEditorToggle = document.getElementById('darkEditorToggle');
const spellcheckToggle = document.getElementById('spellcheckToggle');
const replaceBarToggle = document.getElementById('replaceBarToggle');
const footerContainerToggle = document.getElementById('footerContainerToggle');
const replaceBar = document.getElementById('replaceBar');
const findInput = document.getElementById('findInput');
const replaceInput = document.getElementById('replaceInput');
const matchCaseToggle = document.getElementById('matchCaseToggle');
const wholeWordToggle = document.getElementById('wholeWordToggle');
const regexModeToggle = document.getElementById('regexModeToggle');
const toggleButtons = document.querySelectorAll('.toggle');
const replaceAllButton = document.getElementById('replaceAllButton');
const undoButton = document.getElementById('undoButton');
const replaceFeedback = document.getElementById('replaceFeedback');
const mainContainer = document.querySelector('main');
const editorWrapper = document.getElementById('editorWrapper');
const previewerWrapper = document.getElementById('previewerWrapper');
const editor = document.querySelector('textarea');
const previewer = document.querySelector('iframe');
const footerContainer = document.querySelector('footer');
const characterCount = document.getElementById('characterCount');
const characterLabel = document.getElementById('characterLabel');
const byteCount = document.getElementById('byteCount');
const byteLabel = document.getElementById('byteLabel');
const copyButton = document.getElementById('copyButton');
const copyNotification = document.getElementById('copyNotification');
const numberFormatter = new Intl.NumberFormat();
let previousEditorValue = null;
// ====================
// Reusable Functions
// ====================
// Create a Blob from the editor content
function createContentBlob() {
 return new Blob([editor.value], {
 type: 'text/html; charset=utf-8'
 });
}
// Debounce function
function debounce(callback) {
 let timeout = null;
 return function(delay) {
 clearTimeout(timeout);
 timeout = setTimeout(callback, delay);
 };
}
// Toggle class helper function
function toggleClass(element, className, force) {
 element.classList.toggle(className, force);
}
// Check if a button is active
function isActive(button) {
 return button.classList.contains('active');
}
function deactivate(button) {
 button.classList.remove('active');
}
// Interpret some common escape sequences in the replacement string
function interpretEscapeSequences(str) {
 const escapeMap = {
 '\\n': '\n', // Newline
 '\\r': '\r', // Carriage return
 '\\t': '\t', // Tab
 '\\\\': '\\', // Backslash
 };
 return str.replace(/\\[nrt\\]/g, match => escapeMap[match]);
}
// Format numbers with thousand separators
function separateThousands(number) {
 return numberFormatter.format(number);
}
// ====================
// Core Functions
// ====================
// Create a download URL for the editor content
function createDownloadURL() {
 const blob = createContentBlob();
 downloadLink.href = URL.createObjectURL(blob);
}
// Resize the editor font size
function resizeFont() {
 editor.style.fontSize = `${fontSizeSelector.value}px`;
}
// Resize the editor and previewer panes
function resizeEditor() {
 const editorSizeInputValue = editorSizeInput.value;
 editorWrapper.style.flexGrow = editorSizeInputValue;
 previewerWrapper.style.flexGrow = 100 - editorSizeInputValue;
 editorSizeOutput.value = (editorSizeInputValue / 100).toFixed(2);
}
function updatePlaceholder() {
 findInput.placeholder = isActive(regexModeToggle) ? 'pattern or /pattern/flags' : '';
}
function clearReplaceFeedback() {
 if (replaceFeedback.value) {
 replaceFeedback.value = '';
 }
}
function clearUndoState() {
 if (previousEditorValue !== null) {
 previousEditorValue = null;
 undoButton.disabled = true;
 }
}
// Update the iframe with the editor content
function preview() {
 previewer.replaceWith(previewer); // A fresh iframe to delete JavaScript variables
 const previewerDocument = previewer.contentDocument;
 previewerDocument.write(editor.value);
 previewerDocument.close();
}
// Update character count
function updateCharacterCount() {
 const characterCountValue = editor.value.length;
 characterCount.textContent = separateThousands(characterCountValue);
 characterLabel.textContent = characterCountValue === 1 ? 'character' : 'characters';
}
// Update byte count
function updateByteCount() {
 const blob = createContentBlob();
 const byteCountValue = blob.size;
 byteCount.textContent = separateThousands(byteCountValue);
 byteLabel.textContent = byteCountValue === 1 ? 'byte' : 'bytes';
}
// Update both character count and byte count
function updateDocumentMetrics() {
 if (footerContainer.className !== 'hidden') {
 updateCharacterCount();
 updateByteCount();
 }
}
// ====================
// Debounced Functions
// ====================
const debouncedPreview = debounce(preview);
const debouncedUpdateDocumentMetrics = debounce(updateDocumentMetrics);
const debouncedClearCopyNotification = debounce(function() {
 copyNotification.value = '';
});
// ====================
// Preview Update Dispatcher
// ====================
// Determine and trigger the appropriate preview update based on the selected preview mode
function dispatchPreviewUpdate() {
 const previewSelectorValue = previewSelector.value;
 if (previewSelectorValue === 'Instant') {
 preview();
 } else if (previewSelectorValue !== 'Manual') {
 debouncedPreview(previewSelectorValue);
 }
}
// ====================
// Event Listeners
// ====================
downloadLink.addEventListener('click', createDownloadURL);
downloadLink.addEventListener('contextmenu', createDownloadURL);
fontSizeSelector.addEventListener('change', resizeFont);
runButton.addEventListener('click', preview);
editorSizeInput.addEventListener('input', resizeEditor);
for (const button of toggleButtons) {
 button.addEventListener('click', function() {
 toggleClass(this, 'active');
 if (this === verticalViewToggle) {
 toggleClass(mainContainer, 'vertical');
 } else if (this === darkEditorToggle) {
 toggleClass(editor, 'dark');
 } else if (this === spellcheckToggle) {
 editor.spellcheck = !editor.spellcheck;
 } else if (this === replaceBarToggle) {
 toggleClass(replaceBar, 'hidden');
 } else if (this === footerContainerToggle) {
 toggleClass(footerContainer, 'hidden');
 updateDocumentMetrics();
 } else if (this === matchCaseToggle || this === wholeWordToggle) {
 deactivate(regexModeToggle);
 updatePlaceholder();
 clearReplaceFeedback();
 } else if (this === regexModeToggle) {
 deactivate(matchCaseToggle);
 deactivate(wholeWordToggle);
 updatePlaceholder();
 clearReplaceFeedback();
 }
 });
}
for (const textField of [findInput, replaceInput, editor]) {
 textField.addEventListener('input', function() {
 clearReplaceFeedback();
 if (this === editor) {
 clearUndoState();
 dispatchPreviewUpdate();
 debouncedUpdateDocumentMetrics(500);
 }
 });
}
resetButton.addEventListener('click', function() {
 if (editor.value && editor.value !== editor.defaultValue && !confirm('Are you sure you want to reset the editor content to the default template?\nAll unsaved changes will be lost.')) {
 return;
 }
 fileInput.value = '';
 downloadLink.download = 'template.html';
 editor.value = editor.defaultValue;
 clearReplaceFeedback();
 clearUndoState();
 dispatchPreviewUpdate();
 updateDocumentMetrics();
});
selectButton.addEventListener('click', function() {
 editor.select();
});
fileInput.addEventListener('change', async function() {
 const file = this.files[0];
 if (file) { // Ensure that there's a file to read so Chrome, for example, doesn't run this function when you cancel choosing a new file
 downloadLink.download = file.name;
 editor.value = await file.text();
 clearReplaceFeedback();
 clearUndoState();
 dispatchPreviewUpdate();
 updateDocumentMetrics();
 }
});
replaceAllButton.addEventListener('click', function() {
 const findInputValue = findInput.value;
 const replaceInputValue = replaceInput.value;
 const currentEditorValue = editor.value;
 const isCaseSensitive = isActive(matchCaseToggle);
 const isWholeWord = isActive(wholeWordToggle);
 const isRegex = isActive(regexModeToggle);
 if (!findInputValue) {
 replaceFeedback.value = 'No find term entered';
 return;
 }
 // Prepare pattern and flags for RegExp constructor based on find input and options
 let pattern;
 let flags = 'g'; // Global flag is always needed to replace all
 let regex;
 if (isRegex) {
 const regexLiteralMatch = findInputValue.match(/^\/(.*)\/(.*)$/);
 if (regexLiteralMatch) {
 [, pattern, flags] = regexLiteralMatch;
 if (!flags.includes('g')) {
 flags += 'g';
 }
 } else {
 pattern = findInputValue;
 }
 } else {
 pattern = findInputValue.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
 if (isWholeWord) {
 // Unicode-aware word boundaries (mimics \b) with 'u' flag for non-English characters
 pattern = `(?<![\\p{L}\\p{N}_])${pattern}(?![\\p{L}\\p{N}_])`;
 flags += 'u';
 }
 if (!isCaseSensitive) {
 flags += 'i';
 }
 }
 try {
 regex = new RegExp(pattern, flags);
 } catch (error) {
 replaceFeedback.value = error.message;
 return;
 }
 const matches = currentEditorValue.match(regex);
 if (matches) {
 const matchCount = matches.length;
 previousEditorValue = currentEditorValue;
 // Use function replacement in non-regex mode to avoid interpreting $ patterns
 editor.value = currentEditorValue.replace(regex, isRegex ? interpretEscapeSequences(replaceInputValue) : () => replaceInputValue);
 replaceFeedback.innerHTML = `<span class="number">${separateThousands(matchCount)}</span> ${matchCount === 1 ? 'replacement' : 'replacements'} made`;
 undoButton.disabled = false;
 dispatchPreviewUpdate();
 updateDocumentMetrics();
 } else {
 replaceFeedback.value = 'No matches found';
 }
});
undoButton.addEventListener('click', function() {
 editor.value = previousEditorValue;
 replaceFeedback.value = 'Replacement undone';
 clearUndoState();
 dispatchPreviewUpdate();
 updateDocumentMetrics();
});
copyButton.addEventListener('click', function() {
 navigator.clipboard.writeText(location);
 copyNotification.value = 'Link copied';
 debouncedClearCopyNotification(2000);
});
// Save the current state to localStorage
document.addEventListener('visibilitychange', function() {
 if (document.hidden) {
 const currentState = {
 spellcheckToggle: spellcheckToggle.className,
 verticalViewToggle: verticalViewToggle.className,
 darkEditorToggle: darkEditorToggle.className,
 replaceBarToggle: replaceBarToggle.className,
 footerContainerToggle: footerContainerToggle.className,
 matchCaseToggle: matchCaseToggle.className,
 wholeWordToggle: wholeWordToggle.className,
 regexModeToggle: regexModeToggle.className,
 fontSizeSelector: fontSizeSelector.value,
 previewSelector: previewSelector.value,
 editorSizeInput: editorSizeInput.value,
 findInput: findInput.value,
 replaceInput: replaceInput.value,
 editor: editor.value,
 };
 localStorage.setItem('state', JSON.stringify(currentState));
 }
});
// ====================
// Initialization
// ====================
// Restore the state from localStorage
const restoredState = JSON.parse(localStorage.getItem('state'));
if (restoredState) {
 spellcheckToggle.className = restoredState.spellcheckToggle;
 verticalViewToggle.className = restoredState.verticalViewToggle;
 darkEditorToggle.className = restoredState.darkEditorToggle;
 replaceBarToggle.className = restoredState.replaceBarToggle;
 footerContainerToggle.className = restoredState.footerContainerToggle;
 matchCaseToggle.className = restoredState.matchCaseToggle;
 wholeWordToggle.className = restoredState.wholeWordToggle;
 regexModeToggle.className = restoredState.regexModeToggle;
 fontSizeSelector.value = restoredState.fontSizeSelector;
 previewSelector.value = restoredState.previewSelector;
 editorSizeInput.value = restoredState.editorSizeInput;
 findInput.value = restoredState.findInput;
 replaceInput.value = restoredState.replaceInput;
 editor.value = restoredState.editor;
}
// Update the UI based on the restored state
editor.spellcheck = isActive(spellcheckToggle);
toggleClass(mainContainer, 'vertical', isActive(verticalViewToggle));
toggleClass(editor, 'dark', isActive(darkEditorToggle));
toggleClass(replaceBar, 'hidden', !isActive(replaceBarToggle));
toggleClass(footerContainer, 'hidden', !isActive(footerContainerToggle));
resizeFont();
resizeEditor();
updatePlaceholder();
dispatchPreviewUpdate();
updateDocumentMetrics();

Answer 1

Performance: since the question is labeled among others with javascript and html the source code is intended to be loaded by browsers over HTTP requests thus the page load time could improve by minifying the source code and further more could be improved by declaring the variables/constants with a sole const/let keyword for multiple declarations.

const downloadLink = document.getElementById('downloadLink')
 , fontSizeSelector = document.getElementById('fontSizeSelector')
 , previewSelector = document.getElementById('previewSelector')
 , runButton = document.getElementById('runButton')
 , resetButton = document.getElementById('resetButton')
 , selectButton = document.getElementById('selectButton')
 , fileInput = document.getElementById('fileInput')
 , editorSizeInput = document.getElementById('editorSizeInput')
 , editorSizeOutput = document.getElementById('editorSizeOutput')
 , verticalViewToggle = document.getElementById('verticalViewToggle')
 , darkEditorToggle = document.getElementById('darkEditorToggle')
 , spellcheckToggle = document.getElementById('spellcheckToggle')
 , replaceBarToggle = document.getElementById('replaceBarToggle')
 , footerContainerToggle = document.getElementById('footerContainerToggle')
 , replaceBar = document.getElementById('replaceBar')
 , findInput = document.getElementById('findInput')
 , replaceInput = document.getElementById('replaceInput')
 , matchCaseToggle = document.getElementById('matchCaseToggle')
 , wholeWordToggle = document.getElementById('wholeWordToggle')
 , regexModeToggle = document.getElementById('regexModeToggle')
 , toggleButtons = document.querySelectorAll('.toggle')
 , replaceAllButton = document.getElementById('replaceAllButton')
 , undoButton = document.getElementById('undoButton')
 , replaceFeedback = document.getElementById('replaceFeedback')
 , mainContainer = document.querySelector('main')
 , editorWrapper = document.getElementById('editorWrapper')
 , previewerWrapper = document.getElementById('previewerWrapper')
 , editor = document.querySelector('textarea')
 , previewer = document.querySelector('iframe')
 , footerContainer = document.querySelector('footer')
 , characterCount = document.getElementById('characterCount')
 , characterLabel = document.getElementById('characterLabel')
 , byteCount = document.getElementById('byteCount')
 , byteLabel = document.getElementById('byteLabel')
 , copyButton = document.getElementById('copyButton')
 , copyNotification = document.getElementById('copyNotification')
 , numberFormatter = new Intl.NumberFormat();
let previousEditorValue = null;

replaceAllButton.addEventListener('click', function() {
const findInputValue = findInput.value
 , replaceInputValue = replaceInput.value
 , currentEditorValue = editor.value
 , isCaseSensitive = isActive(matchCaseToggle)
 , isWholeWord = isActive(wholeWordToggle)
 , isRegex = isActive(regexModeToggle);
if (!findInputValue) {
 replaceFeedback.value = 'No find term entered';
 return;
}
// Prepare pattern and flags for RegExp constructor based on find input and options
let pattern
 , flags = 'g' /* Global flag is always needed to replace all */
 , regex;
if (isRegex) {
 const regexLiteralMatch = findInputValue.match(/^\/(.*)\/(.*)$/);
 if (regexLiteralMatch) {
 [, pattern, flags] = regexLiteralMatch;
 if (!flags.includes('g')) {
 flags += 'g';
 }
 } else {
 pattern = findInputValue;
 }
} else {
 pattern = findInputValue.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
 if (isWholeWord) {
 // Unicode-aware word boundaries (mimics \b) with 'u' flag for non-English characters
 pattern = `(?<![\\p{L}\\p{N}_])${pattern}(?![\\p{L}\\p{N}_])`;
 flags += 'u';
 }
 if (!isCaseSensitive) {
 flags += 'i';
 }
}
try {
 regex = new RegExp(pattern, flags);
} catch (error) {
 replaceFeedback.value = error.message;
 return;
}
const matches = currentEditorValue.match(regex);
if (matches) {
 const matchCount = matches.length;
 previousEditorValue = currentEditorValue;
 // Use function replacement in non-regex mode to avoid interpreting $ patterns
 editor.value = currentEditorValue.replace(regex, isRegex ? interpretEscapeSequences(replaceInputValue) : () => replaceInputValue);
 replaceFeedback.innerHTML = `<span class="number">${separateThousands(matchCount)}</span> ${matchCount === 1 ? 'replacement' : 'replacements'} made`;
 undoButton.disabled = false;
 dispatchPreviewUpdate();
 updateDocumentMetrics();
} else {
 replaceFeedback.value = 'No matches found';
}
});

...if javascript versions to support include arrow function support use arrow function when access to this is unneeded, the way the function argument of debounce or the second argument of addEventListener are:

const debouncedPreview = debounce(preview)
 , debouncedUpdateDocumentMetrics = debounce(updateDocumentMetrics)
 , debouncedClearCopyNotification = debounce(() => { copyNotification.value = ''; });

I mention again the purpose is to reduce the dimension of the source code of the page to improve the response time for the HTTP requests.

• Mori: Thank you for the feedback! "the page load time could improve by minifying the source code and further more could be improved by declaring the variables/constants with a sole const/let keyword for multiple declarations" While I understand the benefits of compact declarations for reducing file size, I prefer to keep the declarations on separate lines for better readability and maintainability. I'll consider using minification tools during the build process to optimize performance without compromising code clarity.
• Mori: "if javascript versions to support include arrow function support use arrow function when access to this is unneeded" Done: I replaced seven traditional functions with arrow functions.