new is a keyword and has various other gotchas. The most egregious is probably is the silent but deadly failure when you forget it altogether: function MyURL

new is a keyword and has various other gotchas. The most egregious is probably is the silent but deadly failure when you forget it altogether: function MyURL(loc) { this.location = loc; } // benign: var url1 = new MyURL("http://news.ycombinator.com"); // redirects the user to Hacker News: var url2 = MyURL("http://news.ycombinator.com"); Calling a free function binds the global object (window in the browser) to the implicit this parameter; assigning a URL to window.location is clearly quite an obvious bug, most cases of forgetting new when calling a constructor are much more subtle. Luckily, you can defend against it in the constructor's code: function MyURL(loc) { if (this === (function(){return this;})()) { // might want to log this during development, too... return new MyURL(loc); } this.location = loc; } I strongly recommend reading what Douglas Crockford has written on the topic of JavaScript, or watching his excellent videos. (this from someone who detests video as a medium of conveying this sort of info)