add change assertion - enforce a change to a value

[timruffles]

Hi - initial pull request to see what people think of a change assertion. It's useful to avoid false positives where a test that previously caused an effect doesn't any longer, but this failure is masked by something else having caused it.

addUser();
assert.equal(users.length,1);

This is prone to false positives: if we break addUser() and merge it into a branch where users started with a default user, we'd not get a failing test. With a change assertion we can be more specific about the change we want to see:

assert.change(function() { return users.length},addUser,{by: 1});

Would avoid this issue.

[coveralls]

Coverage remained the same when pulling 7a062a6 on timruffles:add-change-assertion into 0e560c6 on chaijs:master.

[coveralls]

Coverage remained the same when pulling 7a062a6 on timruffles:add-change-assertion into 0e560c6 on chaijs:master.

[logicalparadox]

This is a great idea but would be better implemented as a plugin.

• If you have not implemented a plugin before, chai-spies is a good place to see a project template.
• Once completed and all tests pass open a PR adding your plugin to the official list here.

[timruffles]

Okay - is this the default answer to new assertions? Would be good to note on the docs so people know not to open PRs etc.

[logicalparadox]

Ahh, good point. I updated our wiki with more details and will put together a contributing document for next release.

[timruffles]

Thanks :) Will take a look at adding a plugin.

On Saturday, 30 November 2013, Jake Luer wrote:

Ahh, good point. I updated our wikihttps://github.com/chaijs/chai/wiki#contributingwith more details and will put together a contributing document for next
release.

—
Reply to this email directly or view it on GitHubhttps://github.com//pull/218#issuecomment-29556753
.

[vesln]

Cool idea, though, I wanted to implement this back then. I would be a happy user of this plugin.

[timruffles]

Ok, plugin up! Let me know what you think :)