Text Difference Checker — Diff Checker Online

No — this tool is 100% client-side. All diff computation happens entirely in your browser using JavaScript. Your text is never sent to any server, logged, stored, or seen by anyone. This makes it safe to use with sensitive documents such as contracts, code, medical records, or confidential drafts. The green "🔒 100% Private" badge in the header confirms this. You can verify this by disabling your internet connection — the tool will still work perfectly offline after the page loads.

Side-by-Side shows original text on the left and changed text on the right, with matching lines aligned — additions are highlighted green on the right, deletions are highlighted red on the left. This is best for reviewing structural changes where you want to see both versions simultaneously. Unified (also called inline diff) shows all lines in a single column, with deleted lines marked with a red "−" prefix and added lines with a green "+" prefix — similar to how git diff displays changes. This is compact and efficient for long texts. Word Diff shows the text as natural paragraphs with individual added and deleted words highlighted inline — best for prose editing, contract redlines, and content review.

The similarity percentage is calculated as the ratio of unchanged lines to the total number of lines in both texts. Specifically: similarity = (2 × unchanged_lines) / (total_lines_left + total_lines_right) × 100%. A score of 100% means the texts are identical; 0% means they share no common lines at all. This gives a quick at-a-glance measure of how similar two documents are. The colour of the bar reflects the score — red for low similarity, orange for moderate, green for high.

Ignore Case treats uppercase and lowercase letters as identical — "Hello" and "hello" will be considered the same. Ignore Trailing Whitespace strips spaces and tabs from the end of each line before comparing — useful for code files where editors sometimes add trailing spaces. Ignore Blank Lines removes empty lines from both texts before comparing — useful when comparing documents where line spacing has changed. Ignore Punctuation strips punctuation from both texts — useful for comparing text where punctuation may have been changed (Oxford comma, dashes, etc.) without flagging those as meaningful changes.

The Additions / Deletions tab extracts only the changed lines into two separate panels. The left panel (red) shows every line that was removed from the original text. The right panel (green) shows every line that was newly added in the changed text. Each panel has its own Copy button so you can quickly grab just the additions or just the deletions without needing to select them manually from the full diff. This is useful for writing changelogs, creating patch notes, or quickly summarising what changed between two versions of a document.

The change navigator (Previous / Next buttons) lets you jump between individual changes one at a time. After comparing, a counter shows how many total changes were found (e.g. "12 changes"). Clicking Next › scrolls to the next change and updates the counter to show which change you're currently viewing (e.g. "4 of 12"). Clicking ‹ Previous goes back. This is useful when there are many changes spread across a long document — instead of scrolling manually to find them, you can navigate directly to each one.

This tool uses an LCS (Longest Common Subsequence) algorithm implemented in JavaScript. LCS finds the longest sequence of elements (lines, words, or characters) that appear in the same order in both texts, then marks everything else as added or deleted. This is the same foundational approach used by Unix diff, Git, and most professional diff tools. The algorithm runs entirely in your browser with no server communication. For very large texts (e.g. 50,000+ lines), performance may slow on older devices as LCS is an O(n²) algorithm.