How do I conditionally assign ruby variables to javascript variables?

Question

I have a website where I need a javascript version of the "current user" object along with the ruby version. I have been assigning these variables doing something like this...

Application Controller:
def get_user
    begin
      @current_user = User.find(session[:user_id]) if session[:user_id]
      @current_user_json = @current_user.to_json
    rescue
      session.delete(:user_id)
      @current_user = nil
      @current_user_json = {}
    end
  end

Web Page:
var current_user = null;
current_user_json = '<%= @current_user_json %>';
if(current_user_json != ''){
        current_user = current_user_json.user;
}

Even when there is a current user, I get the current user is undefined. Probably because I am putting the current_user_json assignment around single quotes. However, if I don't put it around single quotes, I'll always get a javascript error when no user is logged in because the syntax is invalid -

current_user_json = ;

I think I am just looking at this completely wrong and there must be a better way. Given that this is probably a common thing to do, I wanted to get other people's opinion on how to create an object in javascript that is a duplicate of the ruby object.

John Feminella · Accepted Answer · 2010-06-13 17:29:47Z

4

JSON is valid Javascript. Consider removing the quotes and just outputting it directly:

current_user_json = <%= @current_user.nil? ? '' : @current_user_json %>;

Better yet, have your controller do all the work instead of putting logic in the view:

@current_user_json = @current_user.nil? ? '{user: null}' : @current_user.to_json 
# ...
current_user_json = <%= @current_user_json %>;

(EDIT: Incorporated Pointy's suggestion below.)

edited Jun 13, 2010 at 17:29

answered Jun 13, 2010 at 16:23

John Feminella

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1 Comment

Pointy Over a year ago

Except for the "nil" case the JSON can't just be empty, or as the question notes there'll be a Javascript syntax error. It should be something like { user: null }.

Amadan · Accepted Answer · 2010-06-13 16:28:42Z

0

You did not specify where you get your to_json from. If you are using the "json" gem, nil.to_json gives "null", which would produce current_user_json = null in your JS - which is valid. If it's some other library which does not do so, the easiest would probably be to override to_json so it produces valid response:

class NilClass
  def to_json
    "null"
  end
end

answered Jun 13, 2010 at 16:28

Amadan

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2 Comments

John Feminella Over a year ago

This is likely not to be a good idea. What if something else depends on the existing behavior of NilClass.to_json?

Amadan Over a year ago

True, something else could break. However, returning an empty string for a null value is likely inconsistent (I suspect [1, nil, 3] is serialized as [1,null,3] and not [1,,3]), and thus a bug which needs fixing. If anything depends on such behaviour, it is almost certainly a hack-around, and thus also waiting to be repaired as soon as the library is fixed. I'd submit a bug report, and fix the library locally until such time as the library gets fixed properly - or use a better library.

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