jestDiff() call in toWarnDev() returns null, making test failure confusing

[gaearon]

Caused by this line:

react/scripts/jest/matchers/toWarnDev.js

Line 52 in 40fa616

jestDiff(normalizedMessage, expectedMessages[0]);

Calling jestDiff with two different strings returns null.

[bvaughn]

I don't think the bug description is correct:

Calling jestDiff with two different strings returns null.

Documentation for jest-diff says:

Generate a string that will highlight the difference between two values with green and red. (similar to how github does code diffing)

The problem we seem to be seeing is caused by this line. It looks like jest-diff just fails to handle single-line strings?

I'm not sure why they check the multiline case like they do. To me, it looks like the return value in all cases should be return diffStrings(a, b, options);

[bvaughn]

Related Jest issue jestjs/jest#5657