liminfoDiff Checker
Free web tool: Diff Checker
About Diff Checker
The Diff Checker is a free online tool for comparing two blocks of text and visualizing the differences between them. Paste your original text on the left and the modified version on the right, click Compare, and instantly see a side-by-side diff with added lines highlighted in green, removed lines in red, and unchanged lines displayed normally. The tool uses the Longest Common Subsequence (LCS) algorithm to produce an accurate, minimal diff.
Text comparison is essential in software development, technical writing, and configuration management. Developers use diff checking to review code changes before committing, compare API response payloads across environments, and verify that refactoring did not alter output. Technical writers compare document revisions to track edits. System administrators diff configuration files to identify unauthorized changes or drift between staging and production environments.
The diff engine runs entirely in your browser. Both text inputs are processed client-side using a dynamic programming LCS implementation, meaning your code, documents, and configuration data are never transmitted to any external server. The tool provides summary statistics showing the exact count of added, removed, and unchanged lines.
Key Features
- Side-by-side diff view with original text on the left and modified text on the right for intuitive comparison
- Color-coded highlighting: green for added lines, red for removed lines, and no highlight for unchanged content
- Line number display on both sides for quick reference when navigating large texts
- LCS (Longest Common Subsequence) algorithm that produces a minimal, accurate diff without false positives
- Summary statistics showing the exact count of added, removed, and unchanged lines at a glance
- Diff prefix markers (+ for additions, - for removals) matching standard unified diff conventions
- Scrollable diff output area with a 500px max-height for handling long documents without page overflow
- Monospace font rendering that preserves whitespace alignment for accurate code and configuration comparison
Frequently Asked Questions
What is a diff and how does this tool work?
A "diff" (short for difference) identifies what changed between two versions of text. This tool splits both inputs into individual lines, then applies the Longest Common Subsequence (LCS) dynamic programming algorithm to find the optimal alignment. Lines present only in the original are marked as removed (red), lines present only in the modified version are marked as added (green), and lines present in both are shown as unchanged. This is the same fundamental approach used by tools like git diff and Unix diff.
How do I compare two texts?
Paste or type the original text into the left textarea and the modified text into the right textarea. Then click the "Compare" button. The diff result appears below, showing a side-by-side view with line numbers, color coding, and addition/removal markers. Summary statistics above the diff show how many lines were added, removed, or unchanged.
What do the colors in the diff output mean?
Green background indicates lines that were added (present in the modified text but not in the original). Red background indicates lines that were removed (present in the original but not in the modified text). Lines with no background color are unchanged and appear identically in both versions. This color convention matches the standard used by GitHub, GitLab, and most code review tools.
Can I use this to compare source code files?
Yes. The tool works with any plain text, including source code in any programming language. The monospace font preserves indentation alignment, and line numbers make it easy to locate changes. You can compare JavaScript, Python, HTML, CSS, SQL, YAML, JSON, or any other text-based format. Simply paste the contents of your files into the two input areas.
What is the LCS algorithm and why does it matter?
LCS (Longest Common Subsequence) is a classic dynamic programming algorithm that finds the longest sequence of lines common to both texts while preserving their order. By identifying what is the same, the algorithm precisely determines what was added or removed. This produces a minimal diff, meaning it shows the fewest number of changes necessary to transform the original into the modified version, avoiding spurious matches that simpler line-by-line comparison would produce.
Is there a size limit for the text I can compare?
There is no hard limit, but the LCS algorithm has O(m*n) time and space complexity where m and n are the number of lines in each text. For texts with a few thousand lines each, the comparison completes instantly. For very large files (10,000+ lines each), there may be a brief computation delay. For extremely large file comparisons, a desktop diff tool like VS Code or Beyond Compare may be more appropriate.
Can I compare configuration files like JSON or YAML?
Absolutely. Paste your JSON, YAML, TOML, INI, or any configuration file content into the two text areas. The diff will highlight every line that differs, making it easy to spot changed values, added keys, or removed sections. For best results, ensure both files use consistent formatting (indentation, key ordering) so the diff focuses on actual content changes rather than formatting differences.
Is my data safe when using this diff tool?
All text comparison happens entirely within your browser. The two texts you paste are processed locally using JavaScript and are never sent to any server, stored in any database, or logged anywhere. This makes the tool safe for comparing sensitive content such as source code, configuration files with credentials, API responses with user data, or internal documentation.