Mocha prints its own diff on assertion errors by stringifyingerr.actual anderr.expected, then passing both strings tojsdiff 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 ajsdiff 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'sassert as well.

Description of the Change

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

Applicable issues

closes#3675

juergba added2 commits

May 24, 2021 11:38

diff: truncate long actual/expected strings

1586b87

tests

af3b2a8

juergba self-assigned this

May 24, 2021

juergba added type: bug

a defect, confirmed by a maintainer

dependencies

Pull requests that update a dependency file

type: performance

Performance improvements

semver-majorimplementation requires increase of "major" version number; "breaking changes" labels

May 24, 2021

juergba added this to thev9.0.0 milestone

May 24, 2021

juergba requested a review froma team

May 24, 2021 14:25

juergba merged commita93d759 intomaster

May 30, 2021

juergba deleted the juergba/diff branch

May 30, 2021 16:29

joielee09 reviewed

Jun 1, 2021

View reviewed changes

test/reporters/base.spec.js

		@@ -164,6 +164,34 @@ describe('Base reporter', function() {
		' \n actual expected\n \n a foobar inline diff\n '
		);
		});

		it("should truncate overly long 'actual' ", function() {
		var actual = '';

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joielee09Jun 1, 2021

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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.

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joshgoebel commentedOct 7, 2021•
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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.☹️