Why does the JSON option matter?

Without it, two documents that differ only in key order or indentation look completely different. Ticking it sorts the keys at every level and pretty-prints both first, so what you see is a difference in the data rather than in the formatting. The order of items in an array still counts, because in JSON it is part of the data.

What does Ignore whitespace do exactly?

It treats any run of spaces and tabs as one space and ignores spaces at the start and end of a line, so re-indented code, tabs swapped for spaces and trailing spaces do not show as changes. Lines that differ only that way are shown as the same, each side as it was written. Blank lines added or removed still count.

How large can the two texts be?

Lines the two sides share at the start and end are set aside first, so what counts is the stretch from the first change to the last, not the length of the file: a long file edited in one place compares quickly, but edits near both ends leave the whole file to compare. If that stretch is too long - roughly 3,000 lines on each side - the comparison is refused with a message rather than freezing the page, and a diff tool on your own machine will serve you better.

Can it compare Word documents or PDFs?

Not directly: it compares plain text. Copy the text out of each document and paste it in, or save each as a .txt file and open that. A paragraph kept on one line, up to a couple of hundred words, is compared word by word as well, so a changed word in it still stands out.

No. Both sides are compared in the page, which matters when the two things you are diffing are API responses or config files with credentials in them.

Stays on your computer How this works

Free diff checker: compare text, code and JSON

Paste or open two versions and see every changed line and word side by side, in your browser

Original

Changed

Ignore whitespace Ignore case Treat as JSON (sort keys and pretty-print first)

Everything runs locally in your browser.

Compare two texts line by line and word by word

Paste or open two versions of anything written in lines - a contract clause, a config file, an API response, a CSV export, two drafts of a letter - and see what changed side by side. Changed lines are tinted, and inside each one the words that differ are marked, struck through on the left and underlined on the right, so a single changed figure in a long line is easy to spot.

Many online diff checkers send what you paste to their server, and some keep it to make a shareable link. Here both sides are compared by this tab, so the text never leaves your device, which matters for contracts, credentials in config files and customer data. It is free to use, with no account and no upload limit, and it runs entirely in your browser: nothing is uploaded, and while the page stays open, a feature you have already used keeps working offline.

What you can do

Compare two blocks of text or code side by side, with line numbers on both sides
See the changed words inside each changed line, not just which lines differ
Open a text file for either side, or drop one onto its box: .txt, .csv, .json, .xml, .md, source code, logs and so on
Ignore whitespace, so re-indented code and trailing spaces do not count as changes
Ignore case, so "Total" and "TOTAL" compare as the same
Compare two JSON documents with their keys sorted and pretty-printed first, so key order and formatting do not count; long integer IDs are kept exact
Windows and Mac/Linux line endings treated alike, so a file saved on another system does not differ on every line
A count of lines added and removed, a note when the two are identical, and Swap to turn the comparison round

How to compare two texts

Paste the original on the left and the changed version on the right, or press Open file… above either box.
Tick Ignore whitespace or Ignore case if those differences do not matter to you. If both sides are JSON, tick Treat as JSON.
Press Compare. Removed lines are tinted red on the left, added lines green on the right, and changed lines show the differing words marked.
Use Swap if you had them the wrong way round.

Questions

Why does the JSON option matter?: Without it, two documents that differ only in key order or indentation look completely different. Ticking it sorts the keys at every level and pretty-prints both first, so what you see is a difference in the data rather than in the formatting. The order of items in an array still counts, because in JSON it is part of the data.
What does Ignore whitespace do exactly?: It treats any run of spaces and tabs as one space and ignores spaces at the start and end of a line, so re-indented code, tabs swapped for spaces and trailing spaces do not show as changes. Lines that differ only that way are shown as the same, each side as it was written. Blank lines added or removed still count.
How large can the two texts be?: Lines the two sides share at the start and end are set aside first, so what counts is the stretch from the first change to the last, not the length of the file: a long file edited in one place compares quickly, but edits near both ends leave the whole file to compare. If that stretch is too long - roughly 3,000 lines on each side - the comparison is refused with a message rather than freezing the page, and a diff tool on your own machine will serve you better.
Can it compare Word documents or PDFs?: Not directly: it compares plain text. Copy the text out of each document and paste it in, or save each as a .txt file and open that. A paragraph kept on one line, up to a couple of hundred words, is compared word by word as well, so a changed word in it still stands out.
Is my text uploaded?: No. Both sides are compared in the page, which matters when the two things you are diffing are API responses or config files with credentials in them.