# Unit test with mock objects

Classes usually interact with a number of other collaborating classes. Mocking frameworks allow you to replace these collaborators in a controlled fashion so that you can isolate the class being tested.

Verifier does not include a built-in mocking library, but you are free to choose one that suits your purposes. However, to illustrate their utility we include a simple example that uses [Mock4JS](https://sourceforge.net/projects/mock4js/).

The `LocalizedValueUnavailableFormatter` builds on other examples and uses the Caplin internationalisation library to translate the "unavailable" message into the locale of the currently logged in user.

```js
novox.LocalizedValueUnavailableFormatter = function(){
  this.sMessage = ct.i18n("value.not.supplied");
};

caplin.implement(novox. LocalizedValueUnavailableFormatter, caplin.core.Formatter);

novox.LocalizedValueUnavailableFormatter.prototype.format = function(vValue) {
  if(vValue === null || vValue === undefined) {
    return this.sMessage;
  } else {
    return vValue;
  }
};
```

The `setUp()` method uses Mock4JS to create a mock instance of the `[caplin.i18n.Translator,opts="nofollow"](https://docs.caplin.com/developer/api/caplin_trader/4/module-br_i18n_Translator)` class. It is configured to return the value "No Value Supplied" when called with the key "value.not.supplied". This mock instance is used in the constructor of the `LocalizedValueUnavailableFormatter` class.

```js
LocalizedValueUnavailableFormatterTest = TestCase("LocalizedValueUnavailableFormatterTest");

LocalizedValueUnavailableFormatterTest.prototype.setUp = function() {
  var sMessageKey = "value.not.supplied";
  this.m_sMessageValue = "No value supplied";

  Mock4JS.addMockSupport(window);
  Mock4JS.clearMocksToVerify();
  var oMockTranslator = mock(caplin.i18n.Translator);
  oMockTranslator.stubs().i18n(sMessageKey).will(returnValue(this.m_sMessageValue));
  window.ct = oMockTranslator.proxy();
  this.m_oFormatter = new novox.ValueUnavailableFormatter();
};

LocalizedValueUnavailableFormatterTest.prototype.test_NullReturnsNotSuppliedMessage = function() {
  var sResult = this.m_oFormatter.format(null);
  assertEquals(this.m_sMessageValue, sResult);
};

LocalizedValueUnavailableFormatterTest.prototype.tearDown = function() {
  this.m_oFormatter = undefined;
};
```