The proper way to write a SharePoint User to a User Field in a SharePoint list

Question

I'm writing a user to a SharePoint list.

I've read that the SharePoint User field has a string like this inside of it: userId;#userLoginName

I've tried formatting in the same way when writing to a User field, for example when I write this string it works: 9;#i:0#.f|membership|[email protected]

But what is strange (to me at least) is that 9;# seems to work or even 9. Even if I don't pass the userLoginName info at all, the loginId seems to be enough for it to recognize which User I'm talking about.

This seems to imply that when writing to the SharePoint User field, you only need the id, and the userLoginName, or, indeed, everything after the loginId is irrelevant.

Am I correct in my reasoning here? Or perhaps there's unexpected consequences if I leave off the userLoginName information?

Answer 1

Your assumption is correct, only User Id is a mandatory property when specifying value for a User field.

But since SP.FieldUserValue object is used to store value of User field, it is recommended to get and set values using this object as demonstrated in the below example:

var ctx = SP.ClientContext.get_current();
var web = ctx.get_web();
var lists = web.get_lists();
var list = lists.getByTitle(listTitle);
var item = list.getItemById(itemId);

var assignedToVal = new SP.FieldUserValue();
assignedToVal.set_lookupId(11);   //specify User Id 
item.set_item(fieldName,assignedToVal);
item.update();

ctx.executeQueryAsync(
    function() {
        console.log('Updated');
    },
    function(sender,args) {
        console.log('An error occurred:' + args.get_message());
    }
);