Assert an increase/decrease by an amount (delta)

[keithamus]

As per #333 and #330, we should be able to assert on the delta of a .changed/.increased/.decreased.

So far (in #333) the proposed syntaxes are:

foo.should.change('bar').by(5);
foo.should.increase('bar').by(5);
foo.should.decrease('bar').by(5);

or

foo.should.change('bar', 5);
foo.should.increase('bar', 5);
foo.should.decrease('bar', 5);

/cc @oveddan @cmpolis @logicalparadox

[lexi-sh]

The former is closer to the rest of the syntax in chai. In my opinion we should always aim for tests to be human readable, and the 5 in the second set just seem like magic numbers. I see some discussion about how it is uncomfortably generic, but if you really think about other assertions I can't think of a way for the by keyword to be used elsewhere.

[timruffles]

both @matthijsgroen's chai-changes and chai-change (disclaimer: mine) plugins support this, along with async assertions (promises in Matthijs' and callbacks in mine).

I guess the change of maintainer explains why the changes feature didn't land in 2013 in #218, I'm not bitter ;)

[keithamus]

Hey @timruffles, sorry about the overlap - wasn't aware of either of these plugins at the time of merging the original PR. As you ant @matthijsgroen both have plugins which cater for this - do you have any input on this issue? Even if that input is "don't do it" 😄.

[timruffles]

I really like the by feature! It's in both the linked plugins.

If I was to make a suggestion, it's that the current implementation in core doesn't support async use-cases, or functional style:

e.g the below - which is bread and butter node webdev - isn't possible in the current implementation:

it('creates a version', function(done) {

    assert.change(function(cb) {
        create({
            type: "plan",
            name: "v2",
        }, cb)
    }, function(cb) {
            Version.count({}, cb);
    }, { 
        callback: done, by: 1,
    });

});

[matthijsgroen]

https://github.com/matthijsgroen/chai-changes#changebydelta

The changes library supports async, automatically if the return value of the when(function() { }) is a promise.

[lucasfcosta]

+1 for the by feature.
I really think having by improves readability, which is one of the main things that makes Chai awesome.

Also, I can't think of a better way to implement it other than having an assertion just for it.
Even if we have it just because of changes, increases and decreases I still think this is a price worth paying.

[keithamus]

Well, as I've said a few times before - I'm here as a steward, not a dictator. So if people want by then I can't argue.

PRs welcome 😄

[lucasfcosta]

Hi, @keithamus, how are you?
Well, I think #607 is done and ready to be merged.
I will be pleased to work on this as soon as the change related changes are ready for a merge. Can I?

PS.: I hope I'm not bothering you, feel free to answer whenever you want 😃

[keithamus]

Feel free to make a PR for this @lucasfcosta. You're not bothering me, its amazing to have such a good contributor eager to contribute more!

[lucasfcosta]

I think we can close this one, @keithamus !
And thanks for the feedback you gave me on the PR #621