Add an assertion that checks how much a value has changed.

[oveddan]

This would be used to check if, when something is invoked, something else is changed by x.

For example:

var someArray = [1, 2, 3];

expect(function(){
  someArray.pop();
}).to.change(someArray, 'length').by(-1);

I'd be glad to take a stab at this.

[keithamus]

Hey again @oveddan! Thanks for the issue!

Could you explain a little further, maybe with more examples, why this should be added? In your example, I don't see why I would do that, when I could just do:

var someArray = [1, 2, 3];
someArray.pop();
expect(someArray.length).to.eql(2);

[cmpolis]

👍 I had been wanting this for a while and was surprised it's not part of the lib... pull request: #333

change and not.change are nice because you don't need to know or save the initial state:

var car = new Car(...);

car.accelerate.should.change(car, 'speed');
car.accelerate.should.increase(car, 'speed');

car.fillGasTank.should.not.change(car, 'oil');
car.addGallonOfGas.should.change(car, 'gas').by(1);

[keithamus]

Alright, I'm sold on it. @cmpolis thanks for the PR! See #333 for continued discussion.

[oveddan]

@cmpolis Thanks for doing this! Yeah @keithamus it is especially useful for testing operations that change the quantity of a persisted value, in particular when you don't know the original value that was changed.

[lucasfcosta]

This one is done.
Implemented at #333 and #621.

More discussion about this was done at #339 too.