How to Compare Two Code Files Online and Spot Every Change

The short answer

Paste the old version into Original Text and the new version into Changed Text in the Text Diff Checker. It lines the two versions up, marks added lines with + in green and removed lines with − in red and struck through, and shows both line numbers for each row. Tick Ignore whitespace when the only difference you do not care about is indentation, for example after wrapping code in an if block.

When a quick online diff is useful

• Checking what a colleague changed in a snippet they pasted into chat.
• Comparing a config file from production with the one in your repository.
• Seeing what an AI assistant or a formatter changed in your function.
• Reviewing two versions of an SQL query or a JSON payload.

If both files are already in a Git repository, git diff is the natural tool. The online checker is for everything else: code in emails, tickets, chat messages and two browser tabs.

How to compare two versions

Open the Text Diff Checker.

Paste the older version in the left box (Original Text) and the newer one in the right box (Changed Text). Each box shows its line count.

Read the summary: + added, − removed and a % similar badge.

Scroll the Comparison Result table. Each row has the original line number, the changed line number, a marker and the line itself.

Tick Ignore case or Ignore whitespace if those differences are noise for your comparison. The result updates immediately.

The comparison runs in your browser, so pasted code is not sent to a server.

Reading the result: an example

Original:

• def total(items):
• result = 0
• for item in items:
• result += item.price
• return result

Changed:

• def total(items, tax=0):
• result = 0
• for item in items:
• if item.active:
• result += item.price
• return result * (1 + tax)

With default settings, the tool reports 4 added, 3 removed, 22% similar. The function signature and the return line show as removed and re-added, which is correct. But result += item.price also shows as removed and added, even though only its indentation changed when it moved inside the new if.

Tick Ignore whitespace and the result becomes 3 added, 2 removed, 38% similar. Now the indented line counts as unchanged, and the real edits stand out: a new parameter, a new condition and a new return expression.

What "changed" means here

The tool compares whole lines. A line where one character changed appears twice: once as removed (the old version) and once as added (the new version). It does not highlight the individual characters within a line, so when a removed and an added line sit next to each other, compare them by eye to find the exact edit.

How lines are matched

The checker finds the longest sequence of lines common to both versions, keeping their order, and treats everything else as added or removed. This means inserting a few lines near the top does not make every later line look different; the unchanged lines below still line up.

When to use the options

Ignore whitespace

With this ticked, leading and trailing spaces are ignored and runs of spaces or tabs inside a line count as a single space. Use it when:

• code was re-indented (tabs to spaces, 2 spaces to 4),
• a block was wrapped in a new if, try or loop,
• a formatter changed alignment.

Do not use it for whitespace-sensitive files where indentation is meaning, such as YAML or Python, if you are specifically checking that indentation is right.

Ignore case

Useful for SQL keywords (select versus SELECT) or HTML tag names. Leave it off for most programming languages, where userId and userid are different names.

The similarity percentage

The badge is the share of rows in the result that are unchanged. It is a rough guide to how much changed, not a quality score. A small edit to a very short snippet can produce a low percentage simply because there are few lines.

Limits to know about

• Line-based only. There is no character-level highlighting inside a line.
• One combined view. Results appear in a single table with both line numbers, not as two separate columns.
• Very long files can be slow. The matching step compares every line of one version with every line of the other, so the work grows quickly with size. For files of many thousands of lines, use a desktop diff tool or git diff --no-index old.txt new.txt, which compares two files even outside a repository.
• Moved blocks. A block moved from the top of a file to the bottom shows as removed in one place and added in another.

A quick review routine

Before you accept someone else's change or paste a new version over an old one:

Run the comparison with default settings to see everything that changed.

Tick Ignore whitespace to hide formatting noise and focus on logic.

For each removed and added pair, check the edit is intended: renamed variables, changed conditions, new return values.

Untick the option again if indentation matters for that language.

Tips for cleaner comparisons

• Format both versions the same way first. If one JSON payload is minified and the other is pretty-printed, every line differs. Run both through the JSON Formatter before comparing.
• Compare like with like. Make sure line endings and encoding come from the same kind of source; pasting from a PDF or a rich-text editor can add invisible characters.
• Trim what you do not need. Paste only the function or section in question, and the result is easier to read.

Comparing plain lists rather than code? See How to Compare Two Lists and Find Missing or Extra Items. For prose such as contracts or essays, How to Compare Two Texts and Find the Differences covers what to look for.