Diff Checker
Compare text, code, JSON, and files side by side. Instantly highlight differences, track additions and deletions, and get smart change summaries.
Side-by-side vs unified diffs
A side-by-side diff (what this tool shows) places the original and modified versions in two synchronized columns, which makes it easy to read both versions in full context — ideal for reviewing document drafts, comparing configuration files, or checking what changed in an API response. A unified diff interleaves removed and added lines in a single column, the compact format used by git diff and code-review tools. Side-by-side is generally easier for humans scanning for meaning; unified is denser for small changes. This tool also lets you pull individual changes from the modified side back into the original, working like a lightweight merge editor.
What people use a diff checker for
- Code review — spot exactly what changed between two versions of a file without committing anything.
- Config drift — compare the config running in production against the one in your repo to find undocumented edits.
- JSON API responses — diff two payloads to see which fields appeared, disappeared, or changed value.
- Document drafts — compare contract or article revisions when the file format has no tracked changes.
- Data debugging — find the one differing line between two exports that "should be identical."
Getting cleaner diffs
Noise hides real changes. Two tricks eliminate most of it: first, pick the right language mode so syntax highlighting makes structural changes obvious; second, use the Format button before comparing — normalizing indentation and key order in JSON, HTML, or JavaScript means the diff shows real differences instead of whitespace shuffling. Because the comparison runs entirely client-side, it's safe to diff API keys, private configs, and unreleased code: nothing you paste ever leaves your machine.
How to use Diff Checker.
- 1
Paste your original text/code in the left pane
- 2
Paste your modified text/code in the right pane
- 3
Select the language mode (Text, JSON, HTML, XML, Source Code) for optimal syntax highlighting
- 4
Click 'Format' to automatically pretty-print messy code or JSON before comparing
- 5
View the differences highlighted side-by-side and read the smart summary card at the top
Why use this tool.
Spot tiny differences in large configuration files instantly
Review code changes locally without needing git or an IDE
Ensure API responses haven't changed unexpectedly
Completely private file comparison entirely in your browser
Clean, distraction-free interface built for speed and focus
About Diff Checker.
True Tool Deck's Diff Checker is a premium, free online tool for comparing text, files, JSON, XML, HTML, and source code. Built with a fast, private, 100% client-side architecture, you can compare sensitive API responses, configuration files, and code changes without ever uploading your data to a server. Our tool goes beyond standard text comparison by offering syntax highlighting for a dozen programming languages, synchronized side-by-side scrolling, and a "Smart Summary" engine that tells you exactly how many additions, deletions, or structural modifications occurred. Whether you're a developer reviewing code, a writer comparing drafts, or a sysadmin debugging config files, our clean, focused interface makes spotting the differences effortless.
Frequently asked questions.
What is a diff checker?
A diff checker (or difference checker) is a tool that compares two pieces of text, code, or data side by side and highlights the exact additions, deletions, and modifications. It's essential for programmers, writers, and analysts.
Are my files uploaded to a server?
No. True Tool Deck's Diff Checker processes everything entirely client-side in your browser. Your sensitive code, JSON, and text data are completely secure and never leave your device.
Can I compare JSON files?
Yes! Our JSON diff checker mode allows you to pretty-print (format) JSON before comparison, ensuring that structural differences are highlighted accurately rather than just formatting differences.
What code languages are supported?
The source code mode supports syntax highlighting for JavaScript, TypeScript, HTML, CSS, XML, Python, Java, Rust, Go, PHP, SQL, and standard JSON.
How does the smart change summary work?
Unlike basic text comparison tools that only show highlighted lines, our tool analyzes the diff output and generates a human-readable summary, calculating total additions, deletions, and specific changes like added JSON properties.