duck.js -- rich matchers with helpful messages on match failure

duck.js allows you to perform assertions on complex objects.
When those assertions fail, duck.js will try to produce helpful error messages.
For instance, suppose you want to assert the same property on an array of objects:

``javascript
var duck = require("duck");
var isArray = duck.isArray;
var hasProperties = duck.hasProperties;

var users = fetchUsers();
duck.assertThat(users, isArray([
hasProperties({name: "Bob"}),
hasProperties({name: "Jim"}),
]));
`

which might produce an error message like:

`
Expected [object with properties {
name: 'Bob'
}, object with properties {
name: 'Jim'
}]
but element at index 0 didn't match:
value of property "name" didn't match:
was 'Jim'
expected 'Bob'
expected object with properties {
name: 'Bob'
}
element at index 1 didn't match:
value of property "name" didn't match:
was 'Bob'
expected 'Jim'
expected object with properties {
name: 'Jim'
}
`

API

The below is a quick reference to the API.
For more examples, take a look at the tests.

$3

Assert that value satifies matcher.

If value satifies matcher, return normally, otherwise throw an
AssertionError describing the mismatch.

$3

If value is a matcher, return that matcher,
otherwise return duck.equalTo(value).

$3

Matcher for deep equality on value.

$3

An object obj matches duck.isObject(matcherObj) if:

* obj matches duck.hasProperties(matcherObj), and
* there is no key that is present in obj but not in matcherObj

Sample usage:

`javascript
duck.isObject({
name: "Bob",
address: duck.isObject({
city: "Cambridge",
county: "UK"
})
})
`

duck.is is called on each value of the matcher object, meaning that the
above is equivalent to:

`javascript
duck.isObject({
name: duck.is("Bob"),
address: duck.isObject({
city: duck.is("Cambridge"),
county: duck.is("UK")
})
})
`

$3

An object obj matches duck.hasProperties(matcherProperties) if,
for each key in matcherProperties, matcherProperties[key].matches(obj[key])

Sample usage:

`javascript
duck.hasProperties({
name: "Bob",
address: duck.hasProperties({
city: "Cambridge",
county: "UK"
})
})
`

duck.is is called on each value of the matcher object, meaning that the
above is equivalent to:

`javascript
duck.hasProperties({
name: duck.is("Bob"),
address: duck.hasProperties({
city: duck.is("Cambridge"),
county: duck.is("UK")
})
})
`

$3

An array blah matches duck.isArray(matcherArray) if:

* blah.length == matcherArray.length, and
* For 0 <= i < array.length, matcherArray[i].matches(blah[i])

Sample usage:

`javascript
duck.isArray([
duck.hasProperties({name: "Bob"}),
duck.hasProperties({name: "Jim"}),
]))
`

duck.is is called on each element of the matcher array, meaning that the
following are equivalent:

`javascript
duck.isArray(["apple", "banana"])

duck.isArray([duck.is("apple"), duck.is("banana")])
`

$3

Each matcher has the following methods:

#### matcher.matches(value)

Return true if value satifies this matcher, false otherwise.

#### matcher.describeMismatch(value)

Generate a string describing why value doesn't satisfy this matcher.
Behaviour is undefined if value actually satisifies the matcher.

#### matcher.matchesWithDescription(value)

Equivalent to:

`javascript
var isMatch = this.matches(value);
return {
matches: isMatch,
description: isMatch ? "" : this.describeMismatch(value)
};
``

Useful if you're likely to want both the boolean and the mismatch description.

#### matcher.describeSelf()

Generate a string describing the matcher.

Thanks

Thanks to Hamcrest for inspiration.