CodeDiff Checker — How-to Guide
日本語 English
A tool that visually highlights the differences between code or text. Even first-time users can get going in 3 steps.
CodeDiff Checker is a free online tool that visually compares two blocks of code or text and highlights the differences. No signup and no install are required — it runs entirely in your browser. Your code is never sent to a server, so you can safely compare proprietary or work code. Just paste two snippets to run a code diff, diff check, or text comparison instantly.
Paste the "reference / sample code" into the left box (sample code),
and "your code" into the right box (my code).
Click the button to display the differences.
Differences are shown visually with colors.
・When you want to compare the answer with your own code while learning to program
・When you want to see what you changed between before and after an edit
・When you want to find changed parts in article or document text
・When you want to check whether pasted code contains stray characters
Here are typical mistake-spotting scenarios where CodeDiff Checker shines, by language.
Spot value differences like border-radius: 50%; vs border-radius: 5px;, or a misspelling of display: flex, at a glance with color-coded highlights on both sides.
Pinpoint forgotten closing tags for <div> or <section>, and attribute typos (class vs clas), at the line and character level.
Compare error-prone details like a const / cosnt typo, missing braces or commas, and wrong function arguments.
In Python, indentation is part of the syntax. Compare against the sample code to detect indentation drift directly as differences.
Check SQL mistakes that are hard to catch before running — comma positions in a SELECT clause, the wrong JOIN type, or a table-name typo — via the diff.
Visualize full-width spaces ( ) accidentally entered via an IME with a □ marker — a classic cause of errors in pasted code.
Compared with desktop diff apps (Meld, WinMerge) or the CLI diff command, an online diff tool needs no installation and works the moment you open it. CodeDiff Checker runs every comparison inside your browser, so your source code is never uploaded to a server. It works on any OS — Windows, Mac, Linux, and mobile — with no setup.
There are many ways to check differences, but CodeDiff Checker is built for "learners who want something they can use casually, in the browser alone."
| Tool | Type | Strength | Beginner-friendly |
|---|---|---|---|
| git diff | CLI | Powerful diff integrated with version control | △ (needs command-line skills and a Git setup) |
| Meld | Desktop | Lightweight, popular cross-platform diff tool | ○ (installation required) |
| WinMerge | Desktop | Supports folder comparison and 3-way merge | ○ (Windows only, installation required) |
| VS Code diff view | Editor | Best for comparing change history of the same file | ○ (but clunky for comparing two separate snippets) |
| Diffchecker.com | Online | Feature-rich; also handles Excel and PDF diffs | △ (unified diff format) |
| CodeDiff Checker | Online | Side-by-side view, character-level highlighting, full-width space detection | ◎ (free, no install, no signup) |
Desktop tools like Meld and WinMerge require installation on Windows or Mac, but CodeDiff Checker needs no install and works in the browser alone. It runs on any OS — Windows, Mac, Linux, and smartphones.
Yes, it is completely free. No account or installation is required — you can use it instantly in your browser.
Because it compares on a text basis, it supports any programming language and text file format — Python, JavaScript, TypeScript, Java, Ruby, PHP, Go, Rust, C/C++, Swift, Kotlin, CSS, HTML, SQL, YAML, Markdown, and more.
No. All diff processing runs entirely in your browser's JavaScript, so your input is never sent to an external server. You can safely compare proprietary code or text containing personal information.
Yes. Full-width spaces (U+3000) accidentally entered via a Japanese IME are visualized with an orange □ marker in the results. A warning banner also appears at the top, so you can see at a glance which side contains them.
git diff is a powerful tool integrated with version control in a Git repository, but it requires command-line operation and is not suited to comparing separate files or pasted snippets. CodeDiff Checker works by simply pasting into the browser and is optimized for comparing your code against a reference.
Yes. Drag and drop a file onto a text editor area and its contents load automatically. It supports text file formats such as HTML, CSS, JS, Python, and SQL.
Yes. It is responsive and works in smartphone and tablet browsers. On narrow screens, the code areas and diff results stack vertically.
All processing happens in your browser, so it depends on your device's performance. A typical laptop or smartphone can display diffs for several thousand lines of code without issues.
Yes — it is designed primarily for beginners. There is no jargon or setup to learn: paste the sample code and your code side by side, press "Check Diff," and the differences are shown in color.