JavaScript: The Definitive Guide, 6th Edition

Execution of JavaScript Programs

There is no formal definition of a program in client-side JavaScript. We can say that a JavaScript program consists of all the JavaScript code in a web page (inline scripts, HTML event handlers, and javascript: URLs) along with external JavaScript code referenced with the src attribute of a <script> tag. All of these separate bits of code share a single global Window object. That means that they all see the same Document object, and they share the same set of global functions and variables: if a script defines a new global variable or function, that variable or function will be visible to any JavaScript code that runs after the script does.

If a web page includes an embedded frame (using the <iframe> element), the JavaScript code in the embedded document has a different global object than the code in the embedding document, and it can be considered a separate JavaScript program. Remember, though, that there is no formal definition of what the boundaries of a JavaScript program are. If the container document and the contained document are from the same server, the code in one document can interact with the code in the other, and you can treat them as two interacting parts of a single program, if you wish. JavaScript in Interacting Windows explains more about the global Window object and the interactions between programs in separate windows and frames.

javascript: URLs in bookmarklets exist outside of any document and can be thought of as a kind of user ...