Code Diff Viewer — Compare Code Side by Side
Compare two code blocks and see additions, deletions, and unchanged lines with an LCS-based diff algorithm.
How it works
- 1Paste or type your text in the input field
- 2Click "Show Diff" — processing happens in your browser
- 3Copy the result or download as a text file
What to do next
About Code Diff Viewer
Code Diff Viewer is shaped around how people actually use developer utility utilities online: open the page, drop in a file, get the result. Compare two code blocks and see additions, deletions, and unchanged lines with an LCS-based diff algorithm. The interface stays out of the way once the work begins so the engine can use the available CPU and memory for the actual transformation.
Behind the controls you see, standard browser APIs is doing the actual developer utility. Formats are detected on load and the engine produces a deterministic output for any given input + options combination — useful when you need to re-run a job and expect identical results.
Code Diff Viewer is a static page plus a client-side engine. The browser does the work; there is no separate backend in the loop for the actual processing. That architecture is why the tool starts immediately, why it does not depend on the load on a remote service, and why running multiple jobs in a row does not slow it down.
Typical users of Code Diff Viewer include site reliability engineers triaging logs, students learning new languages and engineers debugging API payloads. The thread connecting all of them is the same: a focused developer utility task that fits cleanly into a browser tab and benefits from a tool with sensible defaults and minimal setup.
Reach for Code Diff Viewer when you need a predictable result on a single file. The page works on the first visit, the controls are visible without a menu, and the output is delivered the moment the engine finishes.
The only practical limit is the 0 MB per-file ceiling, which keeps the tool responsive across a wide range of devices. Run the tool ten times in a row, run it ten thousand times — it behaves the same way and produces the same quality of result.
Code Diff Viewer is intentionally narrow in what it does, which makes it easy to slot into a longer workflow. Take its output, hand it to whichever next tool fits the job, and Code Diff Viewer stays out of your way until the next time you need it.
Code Diff Viewer is built around steady iteration on a small set of options rather than feature creep. Every additional setting attracts a slightly different audience, but a long settings panel makes the common case slower for everyone. The current controls reflect what users of the tool actually use.
Code Diff Viewer returns the result as a download. If you are running multiple jobs, the output names will not collide as long as the input names differ. You can re-run with different settings as many times as you like; each run produces a fresh file with no caching trickery in between.
Some background on the design choices behind Code Diff Viewer: every option you see on the page is there because a real workflow needs it, and every option that is not shown has been deliberately omitted to keep the common case fast. The bias is toward minimal-but-complete.
Code Diff Viewer produces deterministic output: the same input plus the same options always produces the same result. That predictability matters when the result has to match an upstream specification or be reproducible later.
Tips from users who reach for Code Diff Viewer regularly: process one input first to confirm the settings produce what you expect before committing to a batch; treat the page as the working surface and avoid leaving large jobs running in a backgrounded tab where the browser may throttle JavaScript; and if a particular file fails, check whether the source is intact by opening it in its native viewer — most "tool errors" are actually input errors.
Common gotchas worth flagging: the supported formats are listed in the upload area. The 0 MB ceiling is per-file, not per-session; you can run as many separate jobs as you like, but a single oversized input will be rejected on load.
Open the workspace above to start using Code Diff Viewer. The engine loads on the first interaction so the page itself stays light, and once the tool is warm it processes subsequent jobs quickly. The moment the page is interactive, the tool is ready to do real work on your file.
How it works
- 1Reach the Code Diff Viewer page in your browser to begin.
- 2Drop a developer file onto the upload area, or click to pick one from your device.
- 3Adjust the options to match what you need. Sensible defaults cover the most common case, so you can usually skip this step.
- 4Trigger processing. standard browser APIs reads your input, applies the transformation, and writes the result back into the page.
- 5Download the result. The file is generated in your browser and saved through your normal download flow.
- 6Run additional jobs as needed. The same controls and defaults apply on every run.
Common use cases
- Compare two API responses to spot a regression using Code Diff Viewer.
- Encode binary content for transport in a JSON body.
- Convert between data formats while wiring up an integration.
- Hash a string for a quick reproducibility check.
- Inspect a regex against a test string before committing it.
- Pretty-print a minified blob during incident triage.
- Decode a token to confirm its claims during a debugging session.
- Generate boilerplate from a single specification line.
FAQ
How do I provide two code blocks?
Paste both blocks in the input, separated by a line containing only ---.
What algorithm is used?
Longest Common Subsequence (LCS) for accurate diffs on smaller files, with a fallback for large inputs.
What do the symbols mean?
+ indicates added lines, - indicates removed lines, and lines with no prefix are unchanged.
Can I limit context lines?
Yes — choose 3 or 5 context lines to focus on changes, or show all lines for the full diff.
Is my code private?
Yes — diffing runs entirely in your browser with no uploads.
Does it handle large files?
Files over 10,000 lines use a faster linear diff algorithm instead of the full LCS computation.
How fast is Code Diff Viewer?
Most jobs finish in seconds. Speed scales with input size and with how many CPU cycles your browser tab has available — the engine runs in your browser, so it shares resources with whatever else you have open. For inputs near the 0 MB ceiling, expect anywhere from a few seconds to roughly a minute on a typical laptop. Closing other heavy tabs noticeably speeds things up.
Is Code Diff Viewer keyboard accessible?
Code Diff Viewer uses native HTML controls wherever possible, which means keyboard navigation, focus rings, and screen-reader labels work the way the platform expects. The drop zone accepts files via the keyboard-accessible file picker as well as drag-and-drop, and result downloads use standard browser download flows. If you spot an accessibility gap, Favtoo treats it as a bug worth fixing.
Does Code Diff Viewer reduce quality of the result?
Code Diff Viewer is built to preserve quality wherever the underlying developer format allows it. Operations that are mathematically lossless (e.g. structural transformations, lossless re-encoding) round-trip with no perceptible change. Operations that involve a lossy codec inevitably introduce small artefacts at the byte level, but the defaults aim at the sweet spot where output looks or sounds the same to a normal viewer or listener while still being meaningfully smaller or faster than the input.
Is there a programmatic version of Code Diff Viewer?
Code Diff Viewer is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (standard browser APIs) is open-source and can be used directly from your own code.
How is Code Diff Viewer different from desktop apps that do the same thing?
Desktop apps usually have more advanced features but require installation, maintenance and (often) a licence. Paid online tools are convenient but route your file through their servers and gate downloads behind accounts. Code Diff Viewer sits in between: free, instant, and private, but intentionally narrow in scope. For one-off jobs and the common developer utility operations, it is usually the lowest-friction choice; for highly specialised work, a dedicated app is still the right answer.
Can I use Code Diff Viewer on documents that contain personal data?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
Why did Code Diff Viewer reject my input?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.