What a diff checker does

A diff (short for "difference") compares two blocks of text and shows exactly what changed between them. Lines that are the same stay plain; lines that were added are marked in green with a +, and lines that were removed are marked in red with a −. Instead of re-reading two near-identical documents word by word, you see the handful of lines that actually differ.

Why compare in your browser

The two things people compare are usually a before and an after of something that matters — a contract a client sent back, a config file that used to work, two drafts of a report, a block of code. Many online diff tools upload both versions to a server to compare them. This one doesn't: the comparison runs as JavaScript in your own tab, so neither version leaves your device. Load the page, turn off your internet, and it still works.

What it's good for

Document revisions — see what a reviewer actually changed in a contract or proposal.
Config & code — find the one line that broke a working setup.
Writing — compare two drafts and keep the better wording from each.
Copy-paste checks — confirm two lists or exports are really identical.

Use Ignore whitespace when indentation or trailing spaces changed but the content didn't, and Ignore case when only capitalisation differs.

How the comparison works

The tool lines up the two texts using a longest-common-subsequence match — the same idea behind the "diff" in version-control tools like Git. It finds the largest set of lines that appear, in order, in both versions, then marks everything else as added or removed. That's why a small edit in the middle of a long document shows just that edit, rather than flagging every line after it as changed.

Frequently asked questions

Is my text sent anywhere?

No. Both versions stay in your browser — there is no upload and nothing is stored. That's the point of running it locally, and the airplane-mode test proves it.

Does it compare word by word or line by line?

Line by line. Each line is treated as a unit, which is the most useful view for documents, config and code. If a single word changes, you'll see the old line removed and the new line added.

Can it handle large files?

Yes, within reason — comparison work grows with the size of both texts, so extremely large inputs (many thousands of lines each) may take a moment. Nothing is uploaded, so there's no server size limit.

Tools that pair with this one

Text Cleaner — trim and sort both versions first so only real differences remain.
Case Converter — normalise case before comparing if only capitalisation differs.
JSON Formatter — pretty-print two payloads so a line-by-line diff lines up cleanly.

Everything runs locally in your browser. Keep your own copy of both versions — the tool compares them but doesn't save anything.