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side-by-side · lcs · line numbers · local

Diff two text files side by side

Drop in two text files and instantly see a line-by-line LCS diff — added lines green, removed lines red, line numbers preserved. Runs entirely in your browser.

File Diff
Added
+0
lines
Removed
-0
lines
Unchanged
0
lines
Status
—
load both files
Algorithm: Longest Common Subsequence · 2000 line cap
input · files
Original file
Modified file
output · diff (0)

Drop two text files to see the diff

Quick Start

How to diff two files

Side-by-side line diff using LCS — the same algorithm Git uses for unified patches. Up to 2000 lines per side.

  1. 01

    Load original

    Pick the original file on the left.

    Inputs are treated as UTF-8 throughout the pipeline and validated client-side as you type — nothing dispatches until the value is well-formed.

    leftinpututf-8client-sidevalidated
    left · texta.txt · set
    old
    loaded · parsedclient-side
  2. 02

    Load modified

    Pick the changed file on the right.

    Each toggle re-computes through a memoised hook, so flipping options never resets your input and the UI stays snappy on large inputs.

    rightoptionslivememoisedtoggles
    right · memoisedb.txt · live
    new
    options set · memoisedlive
  3. 03

    Read the diff

    Green = added, red = removed, neutral = unchanged.

    Randomness — where used — comes from crypto.getRandomValues, never Math.random. Heavy work sits in useMemo.

    lcscomputeinstantlocaldeterministic
    diff · instantlive · computed
    +added
-removed
    computed · instantinstant
  4. 04

    Copy or download

    Grab a unified-style patch with +/- prefixes.

    Copy writes through navigator.clipboard and downloads use a Blob — zero network traffic. Same input plus same options always yield the same bytes.

    exportcopydownloadzero-networkreproducible
    ok · copiedready · copied
    Copied
    shipped · ready to shipsuccess
Pro tips

Pro tips for everyday utilities

Reach for crypto.getRandomValues

Cryptographically random for IDs, tokens, salts. Math.random is for animation and games, not security.

Sort IDs lexicographically

UUID v4 has no time ordering. Use ULID or UUID v7 if you need sorted IDs.

Mock data with seeds

Faker-style libraries can be seeded so tests are reproducible. Embed the seed in the fixture file.

Reference cards beat memory

Cheatsheets for Vim, Git, Docker, Linux signals are faster than man pages when you’re mid-flow.

Browser APIs vary

Touch events, gamepads, and clipboard differ across vendors. Feature-detect before use.

Persist in the right place

localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data.

Related toolsText DiffSet OperationsFolder Tree DiffFile HashFile MetadataZIP Inspector
Use Cases

When to diff files

Code review, config audits, doc reviews.

Config drift

Spot what changed between two server configs. Random values come from crypto.getRandomValues — cryptographically secure for tokens, salts, and session IDs. Math.random is deterministic enough for animations but predictable enough to be a security bug.

CSPRNG-backed

Code review offline

Compare two snapshots without git. Pair with hashids, ULID, or UUID v7 when you need both random and sortable — useful for primary keys, public-facing IDs, and event log ordering without a sequence column.

reproducible

Doc revisions

See what an editor changed in a draft. Output is deterministic given the same seed, so fixtures and tests stay reproducible across CI runs and contributor machines. Embed the seed in the fixture file for full replay.

snappy

Migration audits

Verify a script's output across runs. Heavy work runs through useMemo so the UI stays snappy on large inputs. Same input + same options yields identical bytes — the kind of stability snapshot tests rely on.

feature-detected

Build artifacts

Compare two CI bundles line by line. Storage tools persist to the right backend: localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data. Pick the right shelf and quota worries disappear.

deterministic

Translation review

Spot the changes in localized text files. Browser feature detection: clipboard, fullscreen, gamepad, and Web Bluetooth APIs gracefully fall back when unavailable. No surprise white screens on Safari or older Firefox.

i18n
FAQ

Frequently asked questions

Classic LCS (longest common subsequence) — the same approach behind git diff.
LCS is O(n*m). At 2000 lines per side the comparison is instant; beyond that the browser can stall.
It reads them as text, so binary will look like garbled lines. Stick to text files.
Yes — Copy diff produces +/- prefixed lines you can paste into a .diff file.
No. Diffing runs entirely in your browser.
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