jest-diff returns null if one string is multiline but the other is not

Do you want to request a feature or report a bug?

bug

What is the current behavior?

https://repl.it/@sompylasar/jest-diff-multiline-vs-oneline jest-diff@22.1.0

----------- a === "foo\nbar\nbaz"
foo
bar
baz
----------- b === "foo\n"
foo

----------- diff:
- Expected
+ Received

  foo
- bar
- baz
+ 
-----------

----------- a === "foo\nbar\nbaz"
foo
bar
baz
----------- b === "foo"
foo
----------- diff:
null
-----------

----------- a === "foo"
foo
----------- b === "foo\n"
foo

----------- diff:
null
-----------

----------- a === "foo"
foo
----------- b === "foo\nbar"
foo
bar
----------- diff:
null
-----------

What is the expected behavior?

----------- a === "foo\nbar\nbaz"
foo
bar
baz
----------- b === "foo\n"
foo

----------- diff:
- Expected
+ Received

  foo
- bar
- baz
+ 
-----------

----------- a === "foo\nbar\nbaz"
foo
bar
baz
----------- b === "foo"
foo
----------- diff:
- Expected
+ Received

  foo
- bar
- baz
-----------

----------- a === "foo"
foo
----------- b === "foo\n"
foo

----------- diff:
- Expected
+ Received

  foo
+ 
-----------

----------- a === "foo"
foo
----------- b === "foo\nbar"
foo
bar
----------- diff:
- Expected
+ Received

  foo
+ bar
-----------

[SimenB]

I think we should fix this before releasing Jest 23.

whether it will seem like a step backward in developer experience if it reports a failure as follows

We can always improve the error message in Jest afterwards, but making jest-diff behave correctly doesn’t have to be blocked by it, IMO.

[thymikee]

The tests were added later by @pedrottimark to document the behavior. Here’s a rough explanation: https://github.com/jest-community/snapshot-diff/pull/27#discussion_r165079275

All in all, looks like it’s about time to fix this and make it right for a generic use case, not only for expect matchers.