This webpage doesn`t contain any more information about JavaScript, but it is rather interesting all the same.
What I have done is move most of the header part of the web page into a JavaScript file - headers.js, and most of the footers into another JavaScript file - footers.js.
They are then declared in the <head> section of the page HTML code, and called by function name by two JavaScript scripts in the <body> section of the page HTML code.
In addition, a variable is used to hold the title of the page, so the same header.js file can be used by lots of webpages.
So it is a form of client-side dynamic webpage construction , instead of a server side dynamic web page construction which is more normal, such as CMS, SSI, or PHP. I am not sure if it should be used in real life, as the webpage will not work if JavaScript is not enabled on the browser. However it has been an interesting experiment in getting it to work.