Limit the size of 'actual'/'expected' strings before generating a diff

[juergba]

Description

Mocha prints its own diff on assertion errors by stringifying err.actual and err.expected, then passing both strings to jsdiff for generating a diff-patch. For big strings this synchronous process can take several minutes.

jsdiff has some known performance issues, so in mid-term we evtl. should evaluate a jsdiff alternative.
On the other hand it's not a good idea to calculate a diff for two huge strings, even with a faster algorithm. We should limit the size of the input strings, as eg. done by Node's assert as well.

Description of the Change

• no change: stringify err.actual and err.expected
• new: when too big, truncate the resulting strings and add a message ... Lines skipped

Applicable issues

closes #3675

[joielee09]

Hello, I'm trying to contribute to Mocha and before I do I'm trying to get idea of mocha's coding style. I want to know why you used 'var' instea of 'const' or 'let'. Thank you in advance.

[joshgoebel]

Can this be a feature flag please so it can be disabled? I was having NO speed issues before but not my test output is entirely broken (it doesn't show the actual diff at all) because it's a bit over 2000 characters. ☹️