Understanding Git Diffs

Agent tools edit files directly, and the safest way to see what they did is the diff: git diff for unstaged changes, git diff --staged for staged ones, git show for the last commit. Here's a commit an assistant made with the message "Refactor checkout: support more discount codes, tidy naming".

diff --git a/script.js b/script.js
--- a/script.js
+++ b/script.js
@@ -1,23 +1,23 @@
+const DISCOUNTS = { RAYA10: 0.1, MERDEKA20: 0.2 };
+
 const cart = [
   { name: "Kopi O", price: 2.5, qty: 2 },
   { name: "Nasi lemak", price: 6, qty: 1 },
   { name: "Kuih lapis", price: 1.5, qty: 3 },
 ];

-function getTotal(items) {
-  return items.reduce((sum, item) => sum + item.price * item.qty, 0);
+function cartTotal(items) {
+  return items.reduce((sum, item) => sum + item.price, 0);
 }

 function applyDiscount(total, code) {
-  if (code === "RAYA10") {
-    return total * 0.9;
-  }
-  return total;
+  const rate = DISCOUNTS[code.toUpperCase()];
+  return total - total * rate;
 }

 function render() {
   const code = document.querySelector("#code").value;
-  const subtotal = getTotal(cart);
+  const subtotal = cartTotal(cart);
   document.querySelector("#subtotal").textContent = "RM " + subtotal.toFixed(2);
   document.querySelector("#total").textContent = "RM " + applyDiscount(subtotal, code).toFixed(2);
 }

How to read it

--- a/ +++ b/

The file before (a/) and after (b/) the change. Git also prints an index … line with file hashes, left out here.

@@ -1,23 +1,23 @@

A hunk header: this chunk covers 23 lines starting at line 1 in the old file, and 23 lines starting at line 1 in the new one.

- line

Removed: this existed before and is gone now.

+ line

Added: this is new.

line (space)

Context: unchanged, shown so you can see where the change sits.

Review the diff against the message

• For each - line, ask: where did this behaviour go? If it isn't in a + line somewhere, it was deleted. * item.qty vanished in a "naming" change.
• Check the new cases. The old applyDiscount returned total for unknown codes. The new one computes total - total * undefined. What's that?
• Look for new assumptions. code.toUpperCase() assumes code is always a string. The old code didn't.
• Mixed changes are a smell. A rename and a logic change in the same line are easy to approve by accident. Ask for separate commits.

function applyDiscount(total, code) {
  const rate = DISCOUNTS[code.toUpperCase()];
  return total - total * rate;
}

function applyDiscount(total, code) {
  const key = (code || "").trim().toUpperCase();
  const rate = DISCOUNTS[key];
  if (rate === undefined) return total;
  return total - total * rate;
}