README
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:
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.
duck.assertThat(value, matcher)
Assert that value
satifies matcher
.
If value
satifies matcher
, return normally, otherwise throw an
AssertionError describing the mismatch.
duck.is(value)
If value
is a matcher, return that matcher,
otherwise return duck.equalTo(value)
.
duck.equalTo(value)
Matcher for deep equality on value
.
duck.isObject(matcherObj)
An object obj
matches duck.isObject(matcherObj)
if:
obj
matchesduck.hasProperties(matcherObj)
, and- there is no key that is present in
obj
but not inmatcherObj
Sample usage:
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:
duck.isObject({
name: duck.is("Bob"),
address: duck.isObject({
city: duck.is("Cambridge"),
county: duck.is("UK")
})
})
duck.hasProperties(matcherProperties)
An object obj
matches duck.hasProperties(matcherProperties)
if,
for each key
in matcherProperties
, matcherProperties[key].matches(obj[key])
Sample usage:
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:
duck.hasProperties({
name: duck.is("Bob"),
address: duck.hasProperties({
city: duck.is("Cambridge"),
county: duck.is("UK")
})
})
duck.isArray(matcherArray)
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:
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:
duck.isArray(["apple", "banana"])
duck.isArray([duck.is("apple"), duck.is("banana")])
Matcher
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:
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.