chai-changes is an extension to the chai assertion library that provides a set of change-specific assertions.

All assertions use a when mechanism.

Using 'expect':

expect(-> codeThatYieldsAChangedResult).to....when ->
  executeTheCodeThatCausesTheChange()

The code within the expect section will be executed first, then the code in the when section will be executed and then the code in the expect section will be executed again and the differences will be asserted.

Same test using 'should':

(-> codeThatYieldsAChangedResult).should....when ->
  executeTheCodeThatCausesTheChange()

Executes the build up pre- and post-conditions. First it executes the pre-conditions, then it will execute the provided callback. And after the callback it will assert the post-conditions. It will change the object in the assertion chain to the result of the callback.

When the callback returns a promise, the post-conditions are executed when the promise is fulfilled.

The when statement will return an assertion promise when the result of the when block is a promise. To use in mocha, you can specify an notify key to trigger done()

expect(-> value).to.change.when(
  -> promise
  notify: done
)

if you use mocha-as-promised, you don't need to specify this notify method

an alternative would be:

expect(-> value).to.change.when(-> promise).then
  -> done()
  (error) -> done(error)

Assert if the 'expect/should' changes its outcome when 'when' is executed

result = 0
(-> result).should.change.when -> result += 1
expect(-> result).to.change.when -> result -= 1
expect(-> result).not.to.change.when -> result = result * 1

Assert if the change of the 'expect/should' has the provided delta

result = 0
(-> result).should.change.by(3).when -> result += 3
expect(-> result).not.to.change.by(-3).when -> result += 1
expect(-> result).to.change.by(-2).when -> result -= 2

Assert if the change starts from a certain value. The value is compared using a deep equal.

result = ['a', 'b']
(-> result).should.change.from(['a', 'b']).when -> result.push('c')
(-> result).should.change.from(['a', 'b']).to(['a', 'b', 'c']).when -> result.push('c')

Assert if the change ends in a certain value. The value is compared using a deep equal.

result = ['a', 'b']
(-> result).should.change.to(['a', 'b', 'c']).when -> result.push('c')
(-> result).should.change.from(['a', 'b']).to(['a', 'c']).when -> result = ['a', 'c']

Assert if the value increases when an action is performed

result = 0
expect(-> result).to.increase.when -> result += 1
expect(-> result).not.to.increase.when -> result
expect(-> result).not.to.increase.when -> result -= 1

Assert if the value decreases when an action is performed

result = 0
expect(-> result).to.decrease.when -> result -= 1
expect(-> result).not.to.decrease.when -> result
expect(-> result).not.to.decrease.when -> result += 1

Assert if the value has a minimal change of 'amount'

result = 0
expect(-> result).to.change.by.atLeast(4).when -> result += 5
expect(-> result).to.change.by.atLeast(4).when -> result -= 10

Assert if the value has a maximum change of 'amount'

result = 0
expect(-> result).to.change.by.atMost(7).when -> result += 5
expect(-> result).to.change.by.atMost(14).when -> result -= 10

Do an npm install chai-changes to get up and running. Then:

var chai = require("chai");
var chaiChanges = require("chai-changes");

chai.use(chaiChanges);

You can of course put this code in a common test fixture file; for an example using Mocha

Chai Changes supports being used as an AMD module, registering itself anonymously (just like Chai). So, assuming you have configured your loader to map the Chai and Chai Changes files to the respective module IDs "chai" and "chai-changes", you can use them as follows:

define(function (require, exports, module) {
    var chai = require("chai");
    var chaiChanges = require("chai-changes");

    chai.use(chaiChanges);
});

If you include Chai Changes directly with a <script> tag, after the one for Chai itself, then it will automatically plug in to Chai and be ready for use:

<script src="chai.js"></script>
<script src="chai-changes.js"></script>

Copyright (c) 2012 Matthijs Groen

MIT License (see the LICENSE file)