Text Diff & Compare

What is Text Diff?

// Two strings that look 'mostly the same' but a naïve check disagrees
const a = 'hello world';
const b = 'hello, world!';

// Character equality
a === b; // false — but only 3 characters actually changed.

// LCS-style diff (this tool, at line + word granularity)
// → 1 line modified, inline highlight: 'hello[, ]world[!]'
// → unified patch:
//   --- original
//   +++ modified
//   @@ -1 +1 @@
//   -hello world
//   +hello, world!

Key Features

Examples

function getUser(id) {
  const u = db.users.find(x => x.id === id);
  return u;
}

function getUser(userId) {
  const u = db.users.find(x => x.id === userId);
  return u;
}

1. The service is provided as-is.
2. Either party may terminate with 30 days notice.
3. Disputes are resolved in California courts.

1. The service is provided as-is.
2. Either party may terminate with 30 days notice.
2a. Termination notice must be in writing.
3. Disputes are resolved in California courts.

GET /api/users 200 14ms
POST /api/orders 201 88ms
GET /api/orders/42 200 21ms

GET /api/users 200 14ms
POST /api/orders 201 4200ms
GET /api/orders/42 500 21ms

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  border: none;

  margin: 0;
  padding: 0;
  border: none;

How to Use

1. 1

   Paste both versions

   Original text on the left, modified text on the right. Live diff renders as you type; large inputs (>200 KB combined) switch to a manual Diff button.

2. 2

   Turn on the ignore options you need

   Ignore case, ignore all whitespace, ignore trailing spaces, or ignore blank lines — each independent, persisted between visits.

3. 3

   Read the diff or grab the patch

   Side-by-Side for human review, Unified for the `---/+++/@@` patch format. Copy unified diff sends a clean patch to your clipboard for code review or `patch -p1`.

Common Diff Pitfalls

Diff: 200 modifications (all because of trailing \r)

Ignore trailing spaces / tabs → 2 real changes

Diff: 87 modifications (all are indent changes)

Ignore all whitespace → 4 actual changes

Diff: paragraph 2 'completely changed' (one blank line moved)

Ignore blank lines → no changes in paragraph 2

0 differences shown, but `cmp` says the files differ

Disable all Ignore options → real diff appears

Text diff on JSON: 100% of lines changed (just a key reorder)

<a href="/tools/json-diff">JSON Diff</a>: 0 differences

Pasted a 20,000-line log — only first 5,000 lines diffed

`diff -u a.log b.log` in terminal handles full file

Common Use Cases

Code review snippet

Drop two versions of a function into the boxes to see a rename, a removed branch, or a new guard clause at a glance. Inline word highlighting is faster than scanning the GitHub diff for a one-line change.

Contract / policy redline

Paste yesterday's contract vs today's revision. Inserted clauses jump out; unchanged paragraphs collapse to gray. Export the unified patch for the legal review trail.

Log timeline investigation

Compare an SRE's pre-incident log slice against the during-incident slice. Latency, status code, and frequency drift surface immediately without `awk`.

Prose / draft revision

Paste a draft and your editor's version. Inline word diff shows exactly which sentences were rewritten — invaluable for accepting or rejecting changes one at a time.

Translation review

Diff an old translation against a re-translation to confirm the new copy preserves meaning, structure, and placeholders. Toggle 'Ignore trailing spaces' to silence noise that translators commonly introduce.

Config / .env file audit

Compare two `.env`, `docker-compose.yaml`, or shell rc files. With 'Ignore blank lines' on, the diff focuses on functional differences instead of formatting churn.

Technical Details

LCS with prefix/suffix trimming

Common leading and trailing lines are stripped before running the dynamic-programming LCS. A 'two 2,000-line configs with one changed line' diff collapses to a 1×1 DP table and renders in under a millisecond.

Token-level intra-line diff

Adjacent remove + add hunks are paired and tokenized via Unicode word/non-word/whitespace runs. A second LCS pass produces the green/red token spans that make modified lines readable.

Unified diff = git/patch compatible

Output follows the `---/+++/@@ -L,C +L,C @@` format defined for GNU patch and used by Git, GitHub, and every code-review tool. Apply with `pbpaste | patch -p0`.

Input cap at 5,000 lines per side

Beyond the cap the diff truncates and warns. For multi-megabyte inputs use command-line `diff -u` or `git diff --no-index` — they stream and handle gigabytes.

Best Practices

Pick the ignore options before reading

Trailing-space, CRLF, and case noise drown out signal. Toggle the right options first; the diff is much easier to read and you stop training yourself to skim past 'fake' changes.

Use Unified when sharing, Side-by-Side when reviewing

Visual columns are for your eyes. Unified patches are for everyone else's terminals. A copied unified diff drops into a Slack message, a Jira comment, or `patch -p1` without translation.

Sanity-check with similarity %

If two files are 'almost the same' but the similarity score is 30%, you have a line-ending or whitespace problem. Toggle 'Ignore all whitespace' and re-check before reading the diff.

Frequently Asked Questions