Diff Checker - Compare Text Online
Compare two text versions side by side and see changes instantly. Line-by-line diff with color highlighting. Find additions, removals, and unchanged lines at a glance. All processing happens in your browser.
About This Tool
The Diff Checker is a free online tool that compares two text documents and highlights the differences between them line by line. It uses a Longest Common Subsequence (LCS) algorithm to accurately identify added, removed, and unchanged lines.
Added lines are highlighted with a green background, removed lines with a red background, and unchanged lines appear without color. Side-by-side line numbers make it easy to trace changes back to the original and modified versions.
All comparison runs entirely in your browser. Your text is never uploaded to any server, making this tool safe for comparing sensitive documents, code, or configuration files offline.
How to Use
- Paste or typeyour original text in the left text area labeled "Original Text".
- Paste or typethe modified version in the right text area labeled "Changed Text".
- Click the Compare button to run the diff algorithm and see the differences.
- Review the results: green lines were added, red lines were removed, and uncolored lines are unchanged.
- Use Swap to exchange the two inputs, Clear Both to start fresh, or Load Sample to see an example comparison.
Frequently Asked Questions
How does the diff algorithm work?
This tool implements the Longest Common Subsequence (LCS) algorithm using dynamic programming. It builds a table to find the longest sequence of lines that appear in both texts in the same order, then uses backtracking to classify each line as added, removed, or unchanged. This is the same approach used by popular tools like diff and version control systems.
Is my text sent to a server?
No. All processing happens entirely in your browser using JavaScript. Your original and changed text never leave your computer, making this tool safe for comparing private documents, source code, passwords, configuration files, and other sensitive content.
What format should my text be in?
The diff checker works with any plain text. You can paste anything from code snippets, JSON, XML, or CSV to prose, markdown, or log files. The comparison is line-based, so formatting with line breaks yields more meaningful results. For very long files, up to several thousand lines, the comparison runs quickly in most modern browsers.
Can I compare entire files?
Yes, you can paste entire file contents into the text areas. The tool compares the text line by line. For very large files (tens of thousands of lines), keep in mind that the LCS algorithm's memory usage is proportional to the product of the line counts, so extremely large inputs may slow down your browser. For typical file sizes, performance is excellent.