Diff output discards assertion details

[bajtos]

When a test failure is printed using the diff output, assertion details are discarded by mocha. This makes it more difficult to understand why the test failed without inspecting test source code. It is also inconsistent with the default (non-diff) algorithm.

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"
*/

Update

Another bit of information that is stripped from the output is the stack trace, as pointed out in the mailing list (link).

expect(false).to.equal(true);
/* result:
  + expected - actual

  +true
  -false
*/

assert.equal(false, true);
/* result:
AssertionError: true == false
 at Context.<anonymous> (/private/tmp/asd/test.js:5:12)
 at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:211:32)
 [etc.]
*/

[bajtos]

I made an attempt to fix this issue via #949, which was rejected.

How about this solution? Always print the assertion message, treat the diff as an extra, nice-to-have detail.

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

I am willing to contribute a patch once the Mocha team decides on what is the acceptable way to fix this bug.

[bajtos]

I have updated the issue description to mention the missing stack trace.

This is my new proposal:

• Optionally print the diff (it available)
• Always print the message+stack trace

1) foo should bar:
    + expected - actual

    +'expected'
    -'actual'

    AssertionError: surname: expected 'actual' to equal 'expected'
     at Context.<anonymous> (/private/tmp/asd/test.js:5:12)
     at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:211:32)
     [etc.]

@travisjeffery what's your opinion?

[SheetJSDev]

Also would like to see this added to the core assert:

var assert = require('assert');
assert.equal('foo', 'bar', 'baz'); // <-- 'baz' is lost

[bajtos]

@SheetJSDev that's just another case of the same underlying problem. 'baz' is stored in AssertionError.message, which is not printed by the diff output.

[busticated]

+1 @bajtos

I just ran into this one as well.

expect(users.length).to.equal(666, 'unexpected user count - NOTE: remember to foo the bar!');
/* result:
   + expected - actual

  +4
  -5
*/

[bajtos]

the situation was partially improved by 34116d5 in v1.14. See also #993 and #991.

[boneskull]

closing as old

[bajtos]

@boneskull I believe the problem was still not fixed :( Are you willing to accept a pull request from me that would fix the issue? The issue is old because I was waiting for input from Mocha maintainers on what's the acceptable solution, since my initial attempt to fix this issue via #949 was rejected.

[clns]

This is still a pretty big problem (with the default reporter at least), especially when using the chai library. For something like this:

expect(1).to.equal(2)

You get:

But if you use a description (e.g. expect(1, 'My description').to.equal(2)), you get:

And it gets even worse when using .not (e.g. expect(1, 'My description').to.not.equal(1)) since there's no diff to output so you will see just the description:

This happens because chai sets the AssertionError.message to My description: expected 1 to not equal 1 and mocha deletes everything except what is before : expected in lib/reporters/base.js.