Jasmine testing pitfall: .join(', ') joins properties that Jasmine has injected into the prototypical inheritance chain

gistfile1.js

// List of badly formatted emails
var _badEmails = ['bad@email', 'roar@me'];

var _validationMessage = 'Invalid emails: ' + badEmails.join(', ');

// _validationMessage should be: 'Invalid emails: bad@email, roar@me'
expect(_validationMessage).toBe('Invalid emails: ' + _badEmails.join(', ')

// However, the expect statement fails as validationMessage becomes:
'Invalid emails: bad@emails, roar@me, function () {
    spyObj.wasCalled = true;
    spyObj.callCount++;
    var args = jasmine.util.argsToArray(arguments);
    spyObj.mostRecentCall.object = this;
    spyObj.mostRecentCall.args = args;
    spyObj.argsForCall.push(args);
    spyObj.calls.push({object: this, args: args});
    return spyObj.plan.apply(this, arguments);
}'

// Note how the jasmine Spy methods have been join together by .join(', ').