code: function extendInterfaces(helperName, helperFactory) {
      interfaceExtensions.push(function () {
          var Mocha = window.Mocha;
          // For all Mocha interfaces (probably just TDD and BDD):
          Object.keys(Mocha.interfaces)
              .forEach(function (interfaceName) {
              // This is the original callback that defines the interface (TDD or
              // BDD):
              var originalInterface = Mocha.interfaces[interfaceName];
              // This is the name of the "teardown" or "afterEach" property for the
              // current interface:
              var teardownProperty = interfaceName === 'tdd' ? 'teardown' : 'afterEach';
              // The original callback is monkey patched with a new one that appends
              // to the global context however we want it to:
              Mocha.interfaces[interfaceName] = function (suite) {
                  // Call back to the original callback so that we get the base
                  // interface:
                  originalInterface.apply(this, arguments);
                  // Register a listener so that we can further extend the base
                  // interface:
                  suite.on('pre-require', function (context, _file, _mocha) {
                      // Capture a bound reference to the teardown function as a
                      // convenience:
                      var teardown = context[teardownProperty].bind(context);
                      // Add our new helper to the testing context. The helper is
                      // generated by a factory method that receives the context,
                      // the teardown function and the interface name and returns
                      // the new method to be added to that context:
                      context[helperName] =
                          helperFactory(context, teardown, interfaceName);
                  });
              };
          });
      });
  }
docs: Registers an extension that extends the global `Mocha` implementation
  with new helper methods. These helper methods will be added to the `window`
  when tests run for both BDD and TDD interfaces.