The line 7 uses the Array() function to declare the webpages array that will contain four elements. The next four lines defines the array elements (the pages URLs). By using the Array() function used without argument allows the insertion of undefined number of elements in the array. The arrindex variable value is randomly generated in the line 6: it will be an integer number situated in the range 0 and 3 because the generated numbers are rounded to the lower value of the closest integer in the range 0 and 3.99.
When the button called Random Page is pushed the randompage() function is called, a random number is generated and based on its value, the array index corresponding to a certain URL is established and in the new browser window opened the web page is shown. The script can be customized in various ways.You can add other URLs by increasing the number of array elements or the script can be adapted to show random content in web pages, such as random quotes.