workers run in another global context that is different from the current
Workers can spawn another worker
Workers can NOT access DOM
workers code must be hosted within the same origin of the parent page
workers can use importScripts() to share functions with others, but remember rule 1, it is in another context
workers can only use message to talk with main thread
importScripts() can add multiple files, they may be downloaded in any order, but will be executed in the order in which you pass the filenames into importScripts() . This is done synchronously;
importScripts() does not return until all the scripts have been loaded and executed.
importScripts() use relative path to current js file,importScripts(relativeToCurrentJs), rather than to general document(new worker(relativePathToDoc))