Is it possible to use javascript to retrieve all users with read permissions to specific list? This (http://www.c-sharpcorner.com/UploadFile/anavijai/how-to-get-all-the-users-from-site-group-in-sharepoint-2013/) shows how to get users from group but what about list. All users in list may not exist in spgoups.

Hi,

Per my understanding, you might want to get a list of users who has Read permission to a specific list using JavaScript.

A solution is that, you can retrieve a list of users within a site collection, check these users permissions to a specific list using JavaScript Client Object Model.

With the help of JavaScript Client Object Model, we can get the effective permissions that the specified user has within a list:

https://msdn.microsoft.com/en-us/library/office/jj838371.aspx

It will return a SP.BasePermissions object, there is a SP.BasePermissions.hasPermissions Method which can Determines whether the current instance has the specified set of permissions:

https://msdn.microsoft.com/en-us/library/office/jj246794.aspx

To retrieve all users within a site collections, code demo in the documentation below would be helpful:

https://msdn.microsoft.com/en-us/library/office/hh185012(v=office.14).aspx

Thanks 

Patrick Liang