Diff Checker

A diff checker is a tool that compares two blocks of text and highlights every difference between them. The word "diff" comes from the Unix diff command, which has been a staple of software development since the early 1970s. At its core, the algorithm finds the longest common subsequence (LCS) shared by both inputs and then marks everything outside that subsequence as either an addition or a deletion. The result is a clear, color-coded view of exactly what changed, what was added, and what was removed.

This tool performs the comparison entirely in your browser using a classic LCS-based diff algorithm — no external libraries, no server round-trips, no data leaving your machine. You paste an original text on the left, a modified version on the right, and click Compare. Additions appear in green, deletions appear in red, and unchanged lines are displayed normally. A summary counter at the top tells you at a glance how many additions, deletions, and unchanged lines exist.

Line-by-Line vs. Word-by-Word Comparison

The default line-by-line mode treats each line as a single unit. If any part of a line changed, the entire line is flagged — this is the same granularity used by Git and most version-control systems, and it works well for structured content like source code, configuration files, and Markdown documents. When you need finer detail, switch to word-by-word mode. In this mode, the diff algorithm runs on individual words within each changed line, so you can pinpoint the exact phrase or variable name that was edited. Word-level diffing is especially helpful for prose, legal documents, and translation review where changes tend to be small edits inside longer paragraphs.

Common Use Cases

• Code review — Compare two versions of a function or module to understand what a colleague changed before approving a pull request.
• Document revision — Writers and editors can paste two drafts of an article, contract, or report to quickly identify every edit that was made between revisions.
• Configuration comparison — System administrators often need to compare nginx.conf, .env, or Kubernetes YAML files across environments. A diff checker instantly reveals the differences that could cause deployment failures.
• Merge conflicts — When Git cannot automatically merge two branches, it presents conflicting sections. Pasting both versions into a diff checker makes it easier to decide which changes to keep.
• Data validation — Compare exported CSV or JSON outputs before and after a database migration to verify that no records were lost or corrupted.

Frequently Asked Questions

How does the diff algorithm work?
The tool uses a longest-common-subsequence (LCS) approach. It builds a matrix comparing every line (or word) in the original text against every line (or word) in the modified text, finds the longest sequence they share in the same order, and then classifies lines outside that sequence as additions or deletions. This is the same foundational technique behind git diff and the Unix diff utility.

What is a merge conflict?
A merge conflict occurs when two branches in a version-control system modify the same section of a file in incompatible ways. The system cannot automatically decide which change to keep, so it marks the region and asks the developer to resolve it manually. Using a diff checker to view both sides of the conflict makes resolution faster and less error-prone.

Can I compare files?
This tool works with text you paste directly into the browser. To compare files, simply open each file in a text editor, copy the contents, and paste them into the Original and Modified panels. Because everything runs client-side, your file contents are never uploaded anywhere.

Is there a size limit?
The tool runs entirely in your browser, so the practical limit depends on your device's available memory. It handles thousands of lines without issue on modern hardware. For extremely large files (tens of thousands of lines), performance may vary by browser.

This diff checker is completely free and runs entirely in your browser. No data is sent to any server — your text never leaves your machine. Use it whenever you need a fast, private way to compare text.