Are there any clear limits of JavaScript in relation to manipulating the browser and DOM?

I heard that getting access to the text a Gmail email is very 开发者_C百科 difficult if not impossible (iframes).

Are there certain areas where JavaScript is not capable of doing something?

iframes won't prevent you from accessing content. JavaScript doesn't really have any limits with regards to manipulating the DOM....it can't, however, access stuff on your computer, or be used to upload files and such. It can't read stuff inside flash files either. You don't really have any choices other than JS anyway.. what kind of road blocks are you anticipating?

Since you've chosen to use firefox-addon tag: no, getting access to Gmail text is unproblematic from an add-on. Doing the same from a regular website however isn't possible unless that website is hosted on mail.google.com. Reason is a security mechanism called same-origin policy. Websites are generally limited by the same-origin policy, add-ons are not.

Different browsers have different limitations that they impose on JavaScript as well as different APIs that they provide to JavaScript to grant it access to different forms of data. Until recently, it was not possible for JavaScript to access local files; however, there are now APIs in some browsers to do this.

There is a concept known as the "same origin" policy that is used to ensure that JavaScript running from the context of one domain or protocol cannot access data from another domain or protocol. However, browser add-ons or extensions can often exempt themselves from these restrictions. Also, some browsers provide APIs specifically for communication between different origins; however, these APIs generally require that this is done with the cooperation and permission of both origins.

From extension JS, you can access any part of Gmail. I wrote a browser extension that allowed me to forward a Gmail email to a Facebook contact. It also appeared in Facebook and allowed me to send Facebook message to Gmail contact. It was so that I didn't need to worry about adding contacts from Google to Facebook and vice versa.

That extension was easy. Once you get passed the iframe piece, it is cake. Good luck!