javascript - How can I test a Backbone subclass in Mocha without duplicating tests for the superclass? -

say have couple of backbone models, rectangle , triangle, each extend polygon.

var polygon = backbone.model.extend({     defaults: {        width: 100,        height: 100,        rotation: 0     },     rotate: function (degrees) {        var oldrotation = this.get('rotation');        var newrotation = (oldrotation + degrees) % 360;        this.set('rotation', newrotation);        return newrotation;     } });  var rectangle = polygon.extend({     area: function (degrees) {       return this.get('width') * this.get('height');     } });  var triangle = polygon.extend({     area: function (degrees) {       return this.get('width') * this.get('height') / 2;     } } 

i want test rectangle , triangle , ensure each of them independently implements rotate correctly, though know (for now) both inherit implementation of rotate polygon.

what don't want create separate unit tests rectangle , triangle exact duplicates of 1 another. in mocha, how can write test rotate , reuse in unit tests both triangle , rectangle?

here unit tests. note duplicate 'rotates 45 degrees' test.

describe('a rectangle', function () {   var r = new rectangle({width: 50, height: 10, rotation: 90});   it('correctly calculates area', function () {     expect(r.area()).to.equal(500);   });   it('rotates 45 degrees', function () {     expect(r.rotate(45)).to.equal(135);   }); });  describe('a triangle', function () {   var t = new triangle({width: 50, height: 10, rotation: 90});   it('correctly calculates area', function () {     expect(t.area()).to.equal(250);   });   it('rotates 45 degrees', function () {     expect(t.rotate(45)).to.equal(135);   }); }); 

the answer obvious after writing out. can create function encapsulate tests polygon.

var describeapolygon = function (name, p) {     describe(name, function () {               it('rotates 45 degrees', function () {         expect(p.rotate(45)).to.equal(135);       });     }); }; 

and use in tests rectangle , triangle.

describe('a rectangle', function () {     var r = new rectangle({width: 50, height: 10, rotation: 90});     describeapolygon('a rectangle', r);     it('correctly calculates area', function () {       expect(r.area()).to.equal(500);     }); });   describe('a triangle', function () {     var t = new triangle({width: 50, height: 10, rotation: 90});     describeapolygon('a triangle', t);     it('correctly calculates area', function () {       expect(t.area()).to.equal(250);     }); }); 

that's cleanest solution i've been able find. i'm curious if else has tackled problem , come different.