Diff Checker compares two blocks of text and highlights exactly what changed, using the Myers diff algorithm — the same algorithm that powers git diff. Line diff mode shows your original and changed text side by side, with added lines highlighted in green and removed lines in red. Word diff mode instead shows a single unified view with individual word-level changes marked inline, which is often easier to scan for small edits within a single paragraph or sentence.

Everything runs locally in your browser as pure text computation — no text you paste is ever uploaded or sent to a server.

Myers' algorithm frames diffing as a shortest-path problem: picture a grid where moving right means "delete a line from the original," moving down means "insert a line from the changed version," and moving diagonally (free) means "this line is unchanged in both." Finding the smallest possible diff becomes finding the shortest path across that grid — which is why the output is a genuinely minimal edit script, not just "any" set of changes that happens to work. See the diff algorithms guide for a deeper walkthrough of how this actually works.

Word diff mode uses the exact same algorithm, just tokenized by word instead of by line — which is why it's the better choice for a single long line (a minified script, a long sentence) where line-level diffing would only ever be able to say "this whole line changed," even if just one word inside it actually did.

This is a plain-text tool by design — paste raw text, code, config files, log excerpts, or anything else that's fundamentally a sequence of characters. If you specifically need to compare two JSON documents by their structure (rather than their literal text, which would flag a reordered key as a change even if the data is identical), the JSON Diff Checker handles that case with a diff algorithm tuned for structured data instead.