Get User object from Person or Group field value using JSOM/JavaScript API

When you access a User field from a list item using the JavaScript Object Model, the value is treated as if it were a lookup field. You can only really invoke fieldValue.get_lookupValue() to get the user's display name and fieldValue.get_lookupId() to get the ID of the user's entry in the site collection user info list.

To get the actual SPUser object, so that you can drill down into the email and login name, you can use the SPWeb.ensureUser() method.

 var clientContext = new SP.ClientContext();
 var list = clientContext.get_web().get_lists().getByTitle("Example Code").getItemById(1);
 clientContext.load(item);
 clientContext.executeQueryAsync(
      function(){ // successfully retrieved value from list item
           var author = item.get_item("Author");
           var user = clientContext.get_web().ensureUser(author.get_lookupValue());
           clientContext.load(user);
           clientContext.executeQueryAsync(
                function(){ // successfully ensured user from user name
                     var email = user.get_email();
                     var login = user.get_loginName();
                     var displayName = user.get_title();
                     alert("User LoginName: " + login + "\nUser Email: " + email + "\nUser Display Name: " + displayName);
                },function(sender,args){ // on error
                     alert(args.get_message());
                }
           );
      },
      function(sender,args){ // on error
           alert(args.get_message());
      }
 );

In this case I would use REST APIs to get the People/Group (Name with presence) column's username by expanding the column e.g. ....$expand=user/group

and then there is an ensureUser Method in the JS Client Object Model:

http://msdn.microsoft.com/en-us/library/ff408786.aspx

Which you could use to create User object.

You can use a code like this:

Below example uses JSOM

SP.SOD.executeFunc("sp.js", "SP.ClientContext", function(){
        SP.SOD.registerSod("sp.userprofiles.js", SP.Utilities.Utility.getLayoutsPageUrl("sp.userprofiles.js"));
        SP.SOD.executeFunc("sp.userprofiles.js", "SP.UserProfiles.PeopleManager", getTaskListItem);
});
        function getTaskListItem(){
        var item;    
            var clientContext = new SP.ClientContext.get_current();

            var web = clientContext.get_web();

            var list = web.get_lists().getByTitle('Tasks'); //replace with your custom list

            item = list.getItemById(1);

            clientContext.load(item);

            clientContext.executeQueryAsync(onSuccess, onFailure);
        }
        function onSuccess(){
        var context = SP.ClientContext.get_current();
        var web = clientContext.get_web();
        var userName = item.get_item("AssignedTo").get_lookupValue();//replace AssignedTo with your person column
        var user = web.ensureUser(userName);
        var email = user.get_email();
    var loginName = user.get_loginName();

        }
        function onFailure(){

            console.log('Failure!');
        }

If you are planning to use REST api, the call would be as below:

https://siteurl/_api/web/lists/getbytitle('Tasks')/items/?$select=ID,Title,Body,Issue/Id,TaskDueDate,Created,AssignedTo/FirstName,AssignedTo/LastName,AssignedTo/Name,AssignedTo/Id,AssignedTo/Email&$expand=Issue/Id,AssignedTo/Id

It can also be used as follows:

here AssignedTo is a person/group column, but you can add you custom person column as well.

var getTasks = function () {
    var promise = $.ajax({
        url: "/_api/web/lists/getbytitle('Tasks')/items/?" + 
            "$select=ID,Title,Body,Issue/Id,TaskDueDate,Created,AssignedTo/FirstName,AssignedTo/LastName,AssignedTo/Name,AssignedTo/Id,AssignedTo/Email&" +
            "$expand=Issue/Id,AssignedTo/Id",
        method: "GET",
        headers: { "Accept": "application/json; odata=verbose" },
    });
    return promise;
};