About Diff Checker
Diff Checker compares two blocks of text line by line and highlights exactly what changed, entirely in your browser. Paste an original version on the left and a modified version on the right — added lines are highlighted in green on the right, removed lines in red on the left, and everything else is left unmarked because it's identical in both. This is the same line-level diffing approach used by version control tools like git, computed locally with nothing you paste ever leaving your browser.
How to use it
- 1
Paste the original text
Paste the starting version into the left pane.
- 2
Paste the modified text
Paste the changed version into the right pane. The diff updates live as you type in either pane.
- 3
Review and copy
Added and removed lines are highlighted directly in each pane. Click Copy Diff to copy a plain-text +/- diff report.
What it does
- Compares two texts line by line using a longest-common-subsequence diff — the same class of algorithm version control tools use
- Highlights removed lines in red on the left and added lines in green on the right
- Shows a live count of additions, removals, and unchanged lines
- Generates a plain-text +/- diff report you can copy elsewhere
- Includes a Swap action to flip which side is 'original' and which is 'modified'
FAQ
Does this diff at the character level or the line level?
Line level — a line is marked as added or removed as a whole, rather than highlighting the specific characters that changed within a line. This keeps the diff fast and easy to read for most text, code, and config comparisons, though it means a one-character edit to a long line shows the whole line as changed rather than just that character.
Why does a line appear as both removed and added instead of 'changed'?
The underlying diff algorithm only knows about matching and non-matching lines — a line that was edited doesn't have a direct 'changed' concept, so it shows up as the old line removed (red, left) and the new line added (green, right). This is standard behavior for line-based diff tools, including git.
Is there a size limit?
The diff algorithm's cost grows with the product of both texts' line counts, so extremely large inputs (for example, two multi-thousand-line files) can be capped with a clear error asking you to compare smaller sections, to avoid freezing the tab. Typical file and code comparisons are well within the limit.
Does this tool upload my text anywhere?
No. Both texts are diffed entirely in your browser using local JavaScript — nothing is sent to a server, logged, or stored.