How to read a git diff without getting confused

A Diff Shows Change, Not State

A diff shows what changed, not what the code currently is. Until you can read a diff fluently, code review is slow and error-prone. The unified diff format is universal — it's what git, GitHub, and every code review tool uses.

Reading the Unified Diff Format

diff --git a/src/auth/login.js b/src/auth/login.js
index 3f2a1b..9e4c2d 100644
--- a/src/auth/login.js    ← old file
+++ b/src/auth/login.js    ← new file
@@ -23,7 +23,9 @@          ← hunk: old start,count → new start,count
 function validateLogin(email, password) {
   if (!email || !password) {
-    return false;                    ← removed line
+    throw new ValidationError(       ← added lines
+      'Email and password required'  ←
+    );                               ←
   }
   return checkCredentials(email, password);
 }

Reading Large Diffs Strategically

Never start at line 1 of a large diff and read to the end. Start with the file list to understand scope. Then read the test changes — they tell you what behavior changed. Then read the implementation changes. Test changes first gives you a behavioral summary before you evaluate the implementation.