Chai: Inconsistent between API guide and message from assert.equal

[sbidolach]

Could you verify why API guide says something different than message from exception ?

Guide:

.equal(actual, expected, [message])

Example:

// test case
assert.equal(result[0].status, 405, 'Response HTTP status equal 405');
// message when assert.equal fire exception
AssertionError: Response HTTP status equal 405: expected 404 to equal 405

[meeber]

@sbidolach Looks like a bug to me.

it("expect equal with comment at start", () => {
  expect(42, "meep").to.equal(43);    // meep
});

it("expect equal with comment at end", () => {
  expect(42).to.equal(43, "meep");    // meep
});

it("assert strictEqual", () => {
  assert.strictEqual(42, 43, "meep"); // meep
});

it("assert equal", () => {
  assert.equal(42, 43, "meep");       // AssertionError: meep: expected 42 to equal 43
});

[lucasfcosta]

Hi @sbidolach, thanks for the issue and thanks @meeber for writing some examples! 😄
I think this problem involves not only Chai, but Mocha too, let me explain why:
When running tests like @meeber described we can really see this difference between the error messages displayed, but that is not due to Chai itself, since it always ends up concatenating the message flag value and the message passed to the assertmethod, as you can see here.

When running tests like this:

it("expect equal with comment at start", () => {
  err(function() {
    expect(42, "meep").to.equal(43);
  }, 'meep')
});

it("expect equal with comment at end", () => {
  err(function() {
    expect(42).to.equal(43, "meep");
  }, 'meep')
});

it("assert strictEqual", () => {
  assert.strictEqual(42, 43, "meep");
  err(function() {
    assert.strictEqual(42, 43, "meep");
  }, 'meep')
});

it("assert equal", () => {
  err(function() {
    assert.equal(42, 43, "meep");
  }, 'meep')
});

Every single one fails and that is because the message really isn't just meep, it is meep: <message_of_the_assert_method_here>.

What makes Mocha show just meep or meep: <message> is the showDiff flag, which, when set to true, shows only meep and the rest gets ignored because the diff is already being shown.

This happens because instead of calling the .equal assertion from the core, the assert.equal assertion works a little bit different, since it does a non-strict comparison (==). So all we have to do is change this:

  assert.equal = function (act, exp, msg) {
    var test = new Assertion(act, msg, assert.equal);

    test.assert(
        exp == flag(test, 'object')
      , 'expected #{this} to equal #{exp}'
      , 'expected #{this} to not equal #{act}'
      , exp
      , act
    );
  };

To this:

  assert.equal = function (act, exp, msg) {
    var test = new Assertion(act, msg, assert.equal);

    test.assert(
        exp == flag(test, 'object')
      , 'expected #{this} to equal #{exp}'
      , 'expected #{this} to not equal #{act}'
      , exp
      , act
      , true
    );
  };

Unfortunately there is no way we can test this, since this is a behavior of the test runner and it involves how it handles showing the test results, its not our own bug.
But since the other assertions have the showDiff flag set to true I think we should also set the showDiff flag to true on this assertion too, just to keep everything consistent.