How to compare code changes safely
A code comparison tool can show what changed, but the reviewer still decides whether those changes are correct. This guide explains how to read a side by side diff and what to check before accepting an update.
Start with the purpose of the change
Before reading the diff, define what the new code is supposed to achieve. A bug fix should usually change the smallest area needed to solve the bug. A feature update may add new logic, options, or interface text. A refactor may change many lines without changing behavior. Knowing the purpose helps you decide whether a difference is expected or suspicious.
When the purpose is unclear, do not approve the change only because the diff looks small. Ask what problem the update solves, what behavior changed, and whether there are tests or examples that prove the result.
Understand added, deleted, and modified rows
An added row appears only in the changed version. It may introduce new validation, a new branch, a new setting, or a new dependency. A deleted row appears only in the original version. It may remove old code, but it may also remove a safety check, a message, or a required configuration. A modified row appears on both sides and usually needs the closest inspection because a small edit can change behavior.
Review modified rows in context. A renamed variable is harmless only if all related references were updated. A changed condition is safe only if the true and false cases are still correct. A changed query or regular expression should be tested with realistic inputs, not only with the example that appears in the diff.
When to ignore whitespace
Ignoring whitespace is useful when code was reformatted by an editor or formatter. It helps you focus on logic changes by reducing noise from indentation, alignment, and spacing. However, whitespace can be meaningful in languages such as Python, YAML, Markdown, and some template formats. In those cases, review with whitespace visible before deciding the change is safe.
If the diff becomes too noisy, compare smaller sections. For example, compare one function at a time instead of a full file that was reorganized. This usually gives a clearer result and reduces the chance of missing an important logic change.
Checks before accepting a change
- Does the changed code match the stated purpose?
- Were validation, permissions, and error handling preserved?
- Were database queries, file paths, or external requests changed?
- Were comments removed that contained important warnings?
- Can the updated code be tested with normal and edge case inputs?
Use the Compare Codes tool to find the differences, then use judgement and testing to decide whether the update should be accepted.