Custom object formatters allow users to customize how an object is
stringified in matcher failure messages. This can already be done by
adding a `jasmineToString` method to the objects in question. But
it's not always desirable or possible to do that, particularly when
objects of a given "type" do not inherit from a specific prototype.
For instance, suppose a web service returns a list of foos that are
deserialized from JSON, e.g.:
{ fooId: 42, /* more properties */ }
The only way to define `jasmineToString` on those is by writing code to
add it to each instance at runtime. But a custom object formatter can
recognize that the object it's looking at is a foo and format it
accordingly:
jasmine.addCustomObjectFormatter(function(obj) {
if (typeof obj.fooId !== 'number') {
return undefined;
}
return '[Foo with ID ' + obj.fooId + ']';
});
Unlike `jasmineToString`, custom object formatters are scoped to a
particular spec or suite and don't require any changes to the code
under test.
This makes it easier to write high quality matchers and asymmetric equality
testers, and is also a step toward supporting custom object formatters.
Previously, Jasmine passed custom object formatters as the second argument
to matcher factories and as and the second argument to asymmetric equality
testers' `asymmetricMatch` method. Matchers and asymmetric equality testers
were responsible for passing the custom object formatters to methods like
`matchersUtil#equals`:
function toEqual(util, customEqualityTesters) {
return {
compare: function(actual, expected) {
// ...
result.pass = util.equals(actual, expected, customEqualityTesters, diffBuilder);
And:
ArrayContaining.prototype.asymmetricMatch = function(other, customTesters) {
// ...
for (var i = 0; i < this.sample.length; i++) {
var item = this.sample[i];
if (!j$.matchersUtil.contains(other, item, customTesters)) {
return false;
}
}
With this change, that is no longer necessary. Matchers and asymmetric
equality testers can ignore the existence of custom equality testers and
still fully support them:
function toEqual(util) {
return {
compare: function(actual, expected) {
// ...
result.pass = util.equals(actual, expected, diffBuilder);
And:
ArrayContaining.prototype.asymmetricMatch = function(other, matchersUtil) {
// ...
for (var i = 0; i < this.sample.length; i++) {
var item = this.sample[i];
if (!matchersUtil.contains(other, item)) {
return false;
}
}
The old interfaces are still supported, for now, but will be deprecated
in a future commit and removed in the next major release after that.
In addition to making matchers and custom equality testers simpler,
this change sets the stage for adding support for custom object
formatters. Those will be architecturally similar to custom equality
testers, and by injecting a `MatchersUtil` instance everywhere we can
add them without requiring user code to pass them around as used to be
the case with custom object formatters.
Steve Gravrock