For example when I access web server likely Javascript will execute.

From there on, how will the browser parse the Javascript, or "execute" the functions, the memory used, etc. How will the browser "handle" all of that?

Does it act like a compilation lexer in that it passes line by line and generates object code, or does it use the DOM, and other specifications to handle memory, etc.

Also, in terms of updating the page, and alterior concurrent executions as well, such as Flash, HTML, Java, etc.

Point be simplified, how does the browser handle the scripts, the memory, and the logic on page from a javascript file?