createTwoFilesPatch and newlines - is this working as expected?

[aliak00]

Not sure if bug or using the api wrong, but trying to get the diff from these two:

Old text

One line
Two line
Other lines

New text with new line

One line
One.Five line
Two line
Other lines

Where new text has a new line at the end of the file, I get this patch diff:

Index: /Users/aliak/dev/quillia/apps/backend/test-app-data.local/README.md
===================================================================
--- /Users/aliak/dev/quillia/apps/backend/test-app-data.local/README.md original
+++ /Users/aliak/dev/quillia/apps/backend/test-app-data.local/README.md modified
@@ -1,3 +1,4 @@
 One line
+One.Five line
 Two line
\ No newline at end of file
-Other lines
+Other lines

The \ No newline at the end of file. Is that in the right place? How do I tell that a new line was added. If I do it the other way around - i.e. new line is in old text, and I remove it in new text then I get

--- /Users/aliak/dev/quillia/apps/backend/test-app-data.local/README.md original
+++ /Users/aliak/dev/quillia/apps/backend/test-app-data.local/README.md modified
@@ -1,3 +1,4 @@
 One line
+One.Five line
 Two line
-Other lines
+Other lines
\ No newline at end of file

Was \ No newline at end of file added or removed?

[ExplodingCabbage]

Hmmm... this looks wrong to me. In the first case (new version has a trailing newline, old version doesn't), GNU diff -u returns this diff:

--- old.txt	2025-04-28 10:39:39.716869201 +0100
+++ new.txt	2025-04-28 10:42:06.787201215 +0100
@@ -1,3 +1,4 @@
 One line
+One.Five line
 Two line
-Other lines
\ No newline at end of file
+Other lines

I would think we ought to be outputting the same patch and the output you exhibit is surely a bug, quite possibly introduced by me since I took over maintaining jsdiff. Yuck. Will investigate.

[ExplodingCabbage]

Oh - but I can't reproduce. This is what I see using the version of jsdiff on master, which agrees with GNU diff:

> old
'One line\nTwo line\nOther lines'
> noo
'One line\nOne.Five line\nTwo line\nOther lines\n'
> diff.createTwoFilesPatch('old.txt', 'new.txt', old, noo)
'===================================================================\n' +
  '--- old.txt\n' +
  '+++ new.txt\n' +
  '@@ -1,3 +1,4 @@\n' +
  ' One line\n' +
  '+One.Five line\n' +
  ' Two line\n' +
  '-Other lines\n' +
  '\\ No newline at end of file\n' +
  '+Other lines\n'

What version were you using that produced the output you quote in the issue?

[ExplodingCabbage]

Aha! I can reproduce the output quoted in the issue if I use diff@5.0.0:

> diff.createTwoFilesPatch('old.txt', 'new.txt', old, noo)
'===================================================================\n' +
  '--- old.txt\n' +
  '+++ new.txt\n' +
  '@@ -1,3 +1,4 @@\n' +
  ' One line\n' +
  '+One.Five line\n' +
  ' Two line\n' +
  '\\ No newline at end of file\n' +
  '-Other lines\n' +
  '+Other lines\n'

But not with diff@7.0.0:

> old = 'One line\nTwo line\nOther lines'
'One line\nTwo line\nOther lines'
> noo = 'One line\nOne.Five line\nTwo line\nOther lines\n'
'One line\nOne.Five line\nTwo line\nOther lines\n'
> diff.createTwoFilesPatch('old.txt', 'new.txt', old, noo)
'===================================================================\n' +
  '--- old.txt\n' +
  '+++ new.txt\n' +
  '@@ -1,3 +1,4 @@\n' +
  ' One line\n' +
  '+One.Five line\n' +
  ' Two line\n' +
  '-Other lines\n' +
  '\\ No newline at end of file\n' +
  '+Other lines\n'

[ExplodingCabbage]

Seems like I am the hero of this story and not the villain as I first assumed! I believe the issue you're hitting is the one I describe fixing in this entry from the release notes for 6.0.0:

• #535 A bug in patch generation functions is now fixed that would sometimes previously cause \ No newline at end of file to appear in the wrong place in the generated patch, resulting in the patch being invalid. These invalid patches can also no longer be applied successfully with applyPatch. (It was already the case that tools other than jsdiff, like GNU patch, would consider them malformed and refuse to apply them; versions of jsdiff with this fix now do the same thing if you ask them to apply a malformed patch emitted by jsdiff v5.)

6.0.0 introduced a new (unrelated) bug and shouldn't be used, but upgrading to 7.0.0 (which is just 6.0.0 without the new bug) should fix everything for you.

[aliak00]

oooh! nice! I will give it a try as soon as I can! but sounds like it should be fine - so ill just ping here again if there is a problem still!

Else, thank you very much :)