Preserve custom messages in diff output

[bajtos]

When the developer adds extra information to assertion, e.g.

expect('actual', 'description').to.equal('expected'); // Chai style

this extra message ('description') is preserved in the output
even in case when Mocha overwrites the original assertion message with
a string diff.

See also chaijs/chai#189.

[logicalparadox]

I'm not sure of what discussion has gone into this but I feel that if we continue to just add properties to AssertionError eventually it will be chaos. For Chai, I don't want to have to worry about properties for different test frameworks to accomplish a given thing.

As such I have started a discussion on what "extras" AssertionError could use and hopefully put together some sort of loose standard that Chai can follow. You can view the discussion at chaijs/assertion-error#1 and I would appreciate any feedback.

@visionmedia I would be particularly interested in your thoughts on the topic. Much thanks!

[bajtos]

@logicalparadox Thanks for a quick reply!

Frankly, there was no discussion about this change so far. I was confused when I found that mocha is swallowing my assert message in certain cases and decided to go ahead and submit a fix. Let's start the discussion now!

To give more context on my problem:

expect(undefined, 'first name').to.equal('expected');
/* result:
AssertionError: first name: expected undefined to equal 'expected'
*/

expect('actual', 'surname').to.equal('expected');
/* result:
actual expected
"expectedactual"
*/

Maybe I was wrong by changing both Mocha and Chai. Perhaps it's enough to fix Mocha so that it always prints the AssertionError message?

What's your opinion, @visionmedia?

[jonahkagan]

I also was surprised to not see a custom message for a failed assert.equal. A workaround I have been using is to translate assert.equal(x, y, message) into assert(x == y, msg).

[hallas]

@logicalparadox whats your opinion now?

[travisjeffery]

issue looks dead, not a fan how this is tacked on AssertionError either

[bajtos]

@travisjeffery Thank you for your comment. The issue seemed to look dead because we were waiting for an opinion from Mocha maintainers.

I filled a proper bug #1065, so that we can discuss the right problem, which is the bug in mocha and not the fix presented here.