How to Compare Code and Format a Diff

When two versions of a file differ, you need to see *exactly* what changed — not scroll two windows side by side hoping to spot it. A code diff highlights added, removed, and changed lines so the difference is obvious at a glance. Whether you are reviewing a change, resolving why something broke, or comparing what a colleague sent against what you have, a diff is the fastest way to understand. This guide explains how diffs work and how to read them.

What a diff shows

A diff compares two blocks of code line by line and marks the changes:

• Added lines — present in the new version, not the old (usually green, marked `+`).
• Removed lines — in the old version, gone in the new (usually red, marked `-`).
• Unchanged lines — shown for context so you know where the changes sit.

Instead of eyeballing two files, you see a single view where only the differences stand out.

How to compare code with this tool

1. Open the Code Diff Formatter tool.
2. Paste the original code on one side and the modified code on the other.
3. View the highlighted differences.

Everything runs in your browser, so your code — often proprietary — is never uploaded.

Reading a diff well

• Focus on the marked lines. Context lines exist only to orient you; the `+` and `-` lines are the story.
• A changed line shows as both a removed old line and an added new line. That is normal — a modification is a delete plus an insert.
• Watch for whitespace-only changes. Sometimes the "difference" is just reformatting — spaces or line endings — with no real logic change. These clutter a diff and are worth recognizing.

When a diff beats other tools

A code diff is for comparing two *versions* of the same thing. If you are checking a text pattern rather than comparing versions, Regex Tester is the tool. For comparing plain prose rather than code, Diff Checker does the general-text version of the same job.

Common uses

• Review a change before applying or committing it.
• Find what broke between a working and a non-working version.
• Compare a colleague's file against yours.
• Verify a merge or paste landed the right lines.

Related tools

To format the code cleanly before comparing — so formatting differences do not muddy the diff — run each side through the matching formatter first, such as JSON Formatter or XML Formatter.

Privacy and limits

Comparison is fully local — no upload, no size limit, nothing stored, offline once loaded. Open the Code Diff Formatter tool to see exactly what changed.