A Jasmine matcher checking if the actual object is an instance of the expected type

jasmine.toBeInstanceOf.js

// Checks if the actual object is an instance of the expected type;
// the functional object `expected` can be any ancestor prototype.
//
// Example:
//     expects(new Backbone.Model()).toBeInstanceOf(Backbone.Model);
jasmine.Matchers.prototype.toBeInstanceOf = function(expected) {
  var actual = this.actual,
      notText = this.isNot ? ' not' : '';
  this.message = function() {
    return 'Expected ' + actual + notText + ' to be an instance of ' + expected;
  }
  return actual instanceof expected;
}