Question 1

How do I compare two texts?

Accepted Answer

Paste the original text into the left box and the modified text into the right box. The diff checker runs automatically and highlights added lines in green, removed lines in red, and changed lines with character-level highlighting. A summary at the top shows how many lines were added, removed and changed, plus a similarity percentage. No upload, no signup — comparison runs entirely in your browser, so the input text never leaves your device.

Question 2

What is a diff checker?

Accepted Answer

A diff checker is a tool that compares two pieces of text and shows which lines (or characters) have been added, removed or changed between them. The concept dates back to 1976 when Hunt and McIlroy published the original algorithm at Bell Labs; modern tools like GNU diff, git diff, and online diff checkers all use variations of the same LCS-based approach. Diff checkers are essential for code review, document editing, configuration management, and anywhere you need to track changes between two versions of plain text.

Question 3

How can I compare two files online?

Accepted Answer

Open a diff checker tool in your browser, paste the contents of both files into the two text boxes, and the diff is computed instantly. For files you have on disk: open them in a text editor, select all (Ctrl+A / Cmd+A), copy (Ctrl+C / Cmd+C), and paste into the diff checker. The tool above accepts any plain-text format — code, JSON, YAML, CSV, logs, Markdown, plain prose. Binary files like Word .docx or PDF must be exported to plain text first because diff tools work on character streams, not binary structures.

Question 4

How do diff tools work?

Accepted Answer

Diff tools split both inputs into lines, then compute the Longest Common Subsequence (LCS) — the longest sequence of lines present in both inputs in the same order. Lines in the LCS are marked equal; lines only in the original are marked removed; lines only in the modified version are marked added. The classic algorithm comes from Hunt and McIlroy (1976); Eugene Myers' O(ND) algorithm (1986) is the modern refinement used by GNU diff and git. After the line diff, many tools run a second character-level diff on adjacent add/remove pairs to highlight what changed within a line.

Question 5

Can I compare code online?

Accepted Answer

Yes — paste the source code of both versions into the diff checker. The tool treats code like any plain text: line-by-line LCS comparison, with per-character highlighting on changed lines. It handles JavaScript, TypeScript, Python, Go, Rust, Java, C/C++, HTML, CSS, SQL, shell scripts, YAML, JSON and any UTF-8 source format. For best results: format both sides with the same code formatter first (Prettier, Black, gofmt) so unrelated whitespace differences don't pollute the diff. Use the 'ignore whitespace' option when comparing reformatted-but-equivalent code.

Question 6

What is the best diff checker?

Accepted Answer

The best diff checker depends on your workflow. For developers in terminal: GNU diff or git diff (built-in, fast, scriptable). For IDE-integrated code review: VS Code's diff view, JetBrains diff or Beyond Compare. For one-off web comparisons: an online diff checker (like the tool above) — no install, instant, runs locally in browser. For privacy-sensitive content (proprietary code, internal docs): pick a browser-based tool that does not upload — the diff checker on this page runs the entire algorithm client-side, so no text leaves your machine.

Question 7

How do I compare JSON files?

Accepted Answer

Paste both JSON documents into the diff checker. Important pre-step: format both sides with the same indent (typically 2 spaces) and sort object keys alphabetically — without normalisation the diff will be polluted by trivial key-order differences. JSON-aware diff tools (jq diff, json-diff) understand the structure semantically and can ignore key order automatically; line-based diff tools require you to normalise first. For small JSON config files, line diff after pretty-printing both sides is usually enough; for large nested structures, consider a dedicated JSON diff library.

Question 8

Can I compare Word documents online?

Accepted Answer

Word .docx files use a binary ZIP format that diff checkers cannot read directly. To compare two Word documents: open each in Microsoft Word, LibreOffice or Google Docs, select all and copy as plain text, then paste into the diff checker. You'll lose formatting (bold, fonts, headings) but you'll catch every text change — additions, deletions, edits, reorderings. For tracked-changes review with formatting preserved, use Word's built-in Review > Compare feature; for a content-only diff, the plain-text approach above is faster and tool-agnostic.

Question 9

How accurate are diff tools?

Accepted Answer

Diff tools are mathematically exact — given two inputs they compute the same LCS-based diff every time, with no probabilistic guesses. Accuracy 'feels' wrong only when the input is poorly normalised: trailing whitespace differences, line-ending differences (LF vs CRLF), tab-vs-spaces inconsistency, or differing key order in JSON. Pre-process both sides with consistent line endings, the same code formatter, and identical key sorting before diffing. The tool above shows the exact LCS diff — if a 'change' surprises you, the input has a difference somewhere; check whitespace and invisible characters first.

Question 10

What formats can be compared?

Accepted Answer

Any UTF-8 plain-text format: source code in any language (JavaScript, Python, Go, Rust, Java, C/C++, HTML, CSS, SQL, shell), JSON, YAML, TOML, XML, CSV, TSV, log files, Markdown, plain prose, configuration files (Apache, Nginx, Dockerfile, .env), even SVG markup. Binary formats — images (PNG, JPG), Word .docx, PDF, Excel .xlsx, executables — cannot be compared by line-based diff tools. For binary content you need format-specific tools (image diff for PNGs, structural Word diff via Microsoft's Compare feature, PDF diff via dedicated PDF tools).

Question 11

What's the difference between line diff and character diff?

Accepted Answer

Line diff treats each line as the unit of comparison — the standard for code, config and structured text. Character diff (also called 'word diff' when split on word boundaries) operates on individual characters within a line — useful for spotting tiny edits in prose, or for showing exactly which substring within a changed line was modified. Most diff tools combine both: line diff identifies which lines changed, then a character sub-diff runs on adjacent add/remove pairs to highlight the in-line changes. Code review tools always use line diff; prose and editorial tools often use word/character diff.

Question 12

How do I detect changes between two versions of code?

Accepted Answer

Paste version A into the left box and version B into the right. The diff checker shows added (green) and removed (red) lines side-by-side with line numbers, plus a summary count and similarity percentage. For ongoing tracking use git diff in your terminal or your IDE's source-control view — git diff produces the same LCS-based diff but tracks history across commits, branches and remote repositories. Use the online tool when you need a one-off comparison: comparing a colleague's pasted snippet to your local file, comparing config from two environments, or reviewing a PR diff outside the GitHub/GitLab UI.

Question 13

Is my text private when using an online diff checker?

Accepted Answer

It depends on the tool. The diff checker on this page runs the entire LCS algorithm client-side in JavaScript — nothing is sent to a server, nothing is logged, nothing leaves your browser. You can verify this by opening DevTools > Network tab and confirming no requests fire when you paste text. Some online diff tools do upload text for processing — check the privacy policy or DevTools network panel before pasting proprietary code, internal documents or any confidential data. For maximum privacy on sensitive content, use a local tool: GNU diff, git diff, or your IDE's diff view — none of these touch the network.

Question 14

What's the difference between unified diff and side-by-side diff?

Accepted Answer

Unified diff is the compact format git, GNU diff and most CLI tools produce: removed lines (prefixed with '-') and added lines (prefixed with '+') interleaved in a single column, with 3 lines of unchanged context above and below each change block. Side-by-side diff (sometimes called 'split view') puts both versions in two columns with changes aligned horizontally — easier for humans to read on small changes, but takes more screen space. Most online diff checkers offer both views; pick side-by-side for visual comparison, unified for compact review or when you need to copy a patch file.

Question 15

How do I compare two CSV files?

Accepted Answer

Paste both CSVs into the diff checker. For a meaningful diff: ensure both files use the same column order, the same line endings (LF vs CRLF), and the same delimiter (comma vs semicolon vs tab). Strip leading/trailing whitespace and ensure header rows match. The line diff will then surface true row additions, deletions and modifications. For very large CSVs (>10,000 rows), specialised tools like csvdiff, daff, or database-driven comparison are faster and produce structured diffs (which row changed in which column). For quick visual checks under a few thousand rows, the line-based diff above is fine.

Question 16

How do I export diff results?

Accepted Answer

Use the 'Copy unified diff' button to get the patch-file format that git apply, GNU patch, and most CI tools accept. The unified format includes file headers, context lines and the +/- markers needed to re-apply the change elsewhere. For sharing a visual side-by-side comparison, take a screenshot or use the embeddable widget if available. To export raw stats only (added/removed/changed counts, similarity %), copy the summary line. Patches are deterministic — the same two inputs always produce the same diff output, so you can confidently share the result with collaborators.

Question 17

What does the LCS algorithm do?

Accepted Answer

LCS = Longest Common Subsequence. Given two sequences (in our case, two arrays of lines), LCS finds the longest ordered subsequence of items that appear in both sequences. Example: LCS of [A, B, C, D] and [A, X, B, D] is [A, B, D] — three lines, in order. Diff tools use LCS to identify which lines were preserved between versions; everything outside the LCS is classified as added or removed. The naïve LCS algorithm is O(N×M) which is fine for files under ~10,000 lines; Eugene Myers' O(ND) algorithm (1986) is faster on inputs with few changes, which is why GNU diff and git use it.

Question 18

How does git diff compare to an online diff tool?

Accepted Answer

git diff and an online diff checker both produce LCS-based diffs — the underlying math is identical. Differences: git diff is built into git, runs on local files, supports comparing commits/branches/staged changes, and integrates with git workflow. An online diff checker is browser-based, takes pasted text rather than file paths, and is best for one-off comparisons (snippets from chat, config from two environments, before/after pasted from a PR). Use git diff for tracked source control work; use an online diff tool when you need to compare arbitrary text without committing it to git.

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