Edit in JSFiddle

var Gallery = function() {
    this.timeout = 3000;
};

Gallery.prototype.setTimeout = function(num) {
    this.timeout = num;
};

var myGallery = new Gallery();
myGallery.setTimeout(5000);

var otherGallery = new Gallery();
otherGallery.setTimeout(1000);

describe("myGallery", function() {
    it("does not copy setTimeout", function() { 
        expect(myGallery.hasOwnProperty("setTimeout")).toBe(false);
    });
    it("has a timeout of 5000", function() {
        expect(myGallery.timeout).toEqual(5000);
    });
});

describe("otherGallery", function() {
    it("does not copy setTimeout", function() { 
        expect(myGallery.hasOwnProperty("setTimeout")).toBe(false);
    });
    it("has a timeout of 1000", function() {
        expect(otherGallery.timeout).toEqual(1000);
    });
});