How to ignore whitespace in a diff and see only the real changes

In the example, a JavaScript function went from 2-space to 4-space indentation, and one line also started multiplying by item.qty. As typed, the tool reports 1 change: 5 lines removed, 5 added, since every indented line differs. With Ignore whitespace on, it reports 1 change: 1 line removed, 1 added. That's the change a reviewer needs to see.

The patch copied with Ignore whitespace on holds only the real edit:

--- a/total.js
+++ b/total.js
@@ -1,7 +1,7 @@
 function total(items) {
   let sum = 0;
   for (const item of items) {
-    sum += item.price;
+        sum += item.price * item.qty;
   }
   return sum;
 }

It's built the way diff -w -u builds one. Its context lines come from the original text, so they keep the old indentation. Apply it and the edited line arrives with its new spacing, while every other line keeps the old one. If you need the new spacing everywhere, copy the patch with Ignore whitespace off.

Git has three levels of this, and it helps to know which one you want:

• git diff -w (--ignore-all-space) ignores all whitespace, even where one line has a space and the other has none. This tool's switch works the same way.
• git diff -b (--ignore-space-change) ignores changes in the amount of whitespace, but a space added where there was none still counts.
• git diff --ignore-space-at-eol only ignores whitespace at the end of lines.

Ignoring all whitespace has a cost. total = a+b and total = a + b become the same line, which is usually fine in code. In Python and YAML, though, indentation is part of the meaning. A line that moved into or out of a block counts as unchanged, so check those files with the switch off before you trust the result.