Diff Checker — Compare Text Online
Compare two texts side by side and highlight differences.
What to do next
Related tools
About Diff Checker
Diff Checker is part of a collection of single-purpose text processing tools. Compare two texts side by side and highlight differences. Each tool is intentionally narrow — it does one thing well rather than offering many overlapping features — which makes the common path predictable and the result easy to verify before you download or copy it.
Diff Checker is shaped for the gap between "I'll do it by hand" and "I'll script it." When the job is small enough that automating it would take longer than doing it, but annoying enough to want a focused tool — that is the situation this page is built for.
The execution path is auditable from the page itself: open developer tools, switch to the Network tab, run a job. The requests you see are static-asset GETs for the engine and the page resources. The actual work is JavaScript code running against the bytes already in your tab's memory.
Technically, the work is done by the jsdiff library, loaded as part of the page. Inputs are recognised automatically and validated before the engine begins processing. Files up to 0 MB are supported per run; that ceiling keeps browser memory usage stable on a wide range of devices.
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.
The heaviest users of Diff Checker tend to be researchers normalising scraped text, support agents standardising replies and developers prepping fixture data. Each group brings slightly different expectations to the tool, but the same single-page architecture serves every one of them with the same response time.
The output handed back by Diff Checker is the output file. If you would prefer to keep the result in the browser instead of downloading it, you can copy it from the result panel and paste it directly into another tab — useful when the next tool in your workflow expects pasted text rather than a file.
Diff Checker 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 Diff Checker stays out of your way until the next time you need it.
Some notes on the design of Diff Checker. The page is intentionally narrow: one input, the controls relevant to the task, and one output. Adding unrelated features would make the common case slower for the majority of users, so the surface is held to what people actually use.
Some background on the design choices behind Diff Checker: 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.
As a single-page tool, Diff Checker stays focused on one text processing step. Multi-step workflows are composed by chaining adjacent tools — each tool produces a standard file the next one can read directly, so a longer pipeline is just a sequence of short tab-and-tab visits.
Tips from users who reach for Diff Checker 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.
Diff Checker is intentionally narrow in scope so the common case is fast and the result is predictable. If you ever need a variation it does not cover, browse the rest of the catalog — there is a good chance an adjacent tool already exists, and switching between tools is just a matter of opening another tab.
How it works
- 1Open the Diff Checker workspace above. The interface is a single page, so there is nothing to navigate.
- 2Drop a text file onto the upload area, or click to pick one from your device.
- 3Pick any non-default settings you need. Most users leave the defaults alone for the first run and only revisit if the result needs tuning.
- 4Trigger processing. the jsdiff library reads your input, applies the transformation, and writes the result back into the page.
- 5Grab the output as soon as the run completes. You can also copy the result instead of downloading if the next tool in your workflow accepts pasted input.
- 6Run additional jobs as needed. The same controls and defaults apply on every run.
Common use cases
- Sort a list of items alphabetically before publishing it using Diff Checker.
- Count the words in a draft to check it fits a brief.
- Diff two drafts of a document side by side.
- Encode user input safely before pasting it into HTML.
- Reformat a JSON blob copied from a log into something readable.
- De-duplicate a list of email addresses pulled from a form export.
- Generate a slug from a long article title.
- Strip messy formatting out of copy pasted from a PDF.
FAQ
How are differences shown?
Added text is highlighted in green, removed text in red. Changes are shown side by side.
Can I compare large texts?
Yes — there is no hard limit. Very large texts may take a moment to diff.
Does it support code comparison?
Yes — it works well for code. Line-by-line diffs are shown with matching indentation.
Can I call Diff Checker from a script?
Diff Checker 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 (the jsdiff library) is open-source and can be used directly from your own code.
Is Diff Checker mobile-friendly?
Diff Checker runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
How long does Favtoo retain my data after using Diff Checker?
Favtoo keeps no copy of your file because Favtoo never receives your file. Diff Checker runs entirely in your browser, the input is held only in your tab's memory, and closing the tab discards it. There is no opt-in cloud history, no "recent jobs" panel synced to an account, and no server-side retention to configure — the architecture simply has nowhere for your file to be stored.
Are there any usage limits on Diff Checker?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run Diff Checker as often as you need; every run produces a full-quality result.
Does Diff Checker require a browser extension or plug-in?
No installation is needed. Diff Checker runs as a normal web page, with no browser extension, no native helper, and no separate desktop client to download. That is partly a privacy choice — extensions can request broad permissions, while a regular page is sandboxed by default — and partly a convenience one: you can use Diff Checker on any computer you have temporary access to without leaving anything installed on it.
Does Diff Checker reduce quality of the result?
Diff Checker is built to preserve quality wherever the underlying text 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.
Does Diff Checker match what professional tools produce?
Diff Checker is built on the jsdiff library, which is the same class of engine used by professional text processing pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.