Compare two versions, keep the best of each

Two texts side by side. Every difference is marked, down to the words. Copy any difference from one side to the other, type in either side, and the table below keeps track of what is still open and what you did.

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Left

Right

changed changed words only in left only in right moved no lines here kept as it is

Open a file on each side, or drop two files anywhere on the page: the first goes to the left, the second to the right. Files are read in this browser; nothing is uploaded.

Differences and changes

#	State	What	Where (line numbers)	Do

Kept on this device only: the two texts, their file names and this table, until you press Forget this session. If the page is open in two tabs, only the newest one saves. Nothing is sent after the page has loaded. To check, open your browser's network log (F12, then Network) after the page has loaded and watch it while you work, or switch the network off: the page keeps working.

How this compares

Method: Line by line first: lines found only once in each text are paired in order and hold the comparison in place (the patience method), the longest runs of shared lines are paired between them, and the Myers method, as in most compare tools, does the rest. What is left over is a difference. So typing or taking in one place does not change how other places are compared (a rare exception: the same lines found in two orders of almost the same length). Inside a changed line, the changed words and letters are marked.
Paired lines: In a changed block, each line is paired with the most similar line on the other side, in order, before the words are marked. A reworded line shows only its changed words; a line with no similar partner is marked whole.
Moved blocks: Lines removed in one place and found again in another are shown as moved, in their own colour, at both places, and listed once in the table. A move needs at least three lines, at least two of them with two characters or more, and 15 characters or more in all (spaces at the start and end of a line not counted), its lines must be nearly all the same in both places, and at least one line or three-line stretch of it must appear only once in each text, so lists of near-identical lines and repeated boilerplate never count as moves. Taking a moved block puts it at its place from the side you take it from and removes it from its old place. The method follows the one published in VS Code's source (MIT licence), with the uniqueness rule added.
Format: Each side's format is found from its first 64 KB (and its file name), when the text arrives and after a large edit, and the side is coloured for it: keys, text, numbers, words like true and null, comments and tags each have one colour. The menu above a side shows what was found (hover for the reason) and lets you choose another. Format both sides rewrites both texts with the same layout (JSON, with or without comments, or JSON Lines; indent, sorted keys, comments and trailing commas as you choose) before comparing, so only real changes are left. Every number and string is copied exactly as written, never rounded. It is one row in the table; its Undo gives both texts back exactly. If a side is not valid, nothing changes and the page says which side and where.
Options: Spaces at line ends, all spaces, or letter case can be ignored. Lines that differ only in those ways then count as equal and are left as they are on each side.
Kept as it is: You looked at a difference and decided to leave both sides as they are. It counts as handled and stays in the table while you take, type or change an option elsewhere. If you later change its lines by hand, it is a new difference and shows as open again; undo that change and it is kept again.
Undo: Every change in the table has its own Undo, even after other changes. If you later typed over the same lines, that row says so and changes nothing: undo the later change first.
Files: Text files in UTF-8, with or without a byte order mark, up to 20 MB. Windows (CRLF) and Unix (LF) line ends are kept per side, shown above the side, and written back the same way when you save. A binary file, or text in another encoding, is refused with a plain message, never shown as garbage.
Limits: Very different texts: after a time limit the page stops looking for the best match and shows larger blocks; it says so above the two sides. Large texts, over 30,000 lines in all (two files of 15,000 lines): compared line by line without moved blocks or marked words, or, with more than about 63,000 different lines, as larger blocks; it says so too. Binary files: refused. One known gap: where the same lines appear in two stretches of almost equal length, in two orders, a “kept” mark can be lost after an edit and must be set again; no text is ever changed by it.
Sent: Nothing. The page compares in your browser and makes no request after it has loaded, also when you open, drop, save or copy files. Your two texts and the table are kept in this browser only, until you press Forget this session.
Open source: CodeMirror 6: @codemirror/view, state, commands, merge, language, lang-json and legacy-modes, with @lezer/common, @lezer/highlight, @lezer/json, @lezer/lr, style-mod, w3c-keyname, crelt and @marijn/find-cluster-break, all MIT licence, (c) Marijn Haverbeke and others. The moved-block method after VS Code's computeMovedLines (MIT licence, (c) Microsoft Corporation), written anew here.

Keys

Alt + ↓ next difference, Alt + ↑ previous, Alt + → take left, Alt + ← take right, Alt + K keep as it is, Ctrl + Z undo, Ctrl + Shift + Z redo. They work while typing in either side.