How works these methods in controllers, assets in js?

Question

I have this example
And i wanna understand what it is " ||= " in this method? How it works?

First segment

 def current_user
            @current_user ||= User.find(session[:user_id]) if session[:user_id]
          end

also, i dont know what does (function()) mean in Ruby on rails. Its not simple function(), why is it inside brackets?

Second example

(function() {
  this.App || (this.App = {});

  App.cable = ActionCable.createConsumer();

}).call(this);

Answer 1

The ||= is equivalent to

@current_user || @current_user = User.find(session[:user_id]) if session[:user_id]

Meaning, if @current_user is nil or false set @current_user to be the value of User.find(session[:user_id]) in the case there is a session[:user_id]

The second example is not Ruby on Rails, but JavaScript. It's the so called “Immediately Invoked Function Expressions" and it's wrapped with parenthesis so that you can call it with a specific parameter, in your case this . So that the this inside the IIFE has the context of where that IIFF was called.

Answer 2

If you think about other operators, it will be easier to see what this memoize does.

When we do @var += 1 , we are basically saying @var = @var + 1 . That's the same for other operators:

@var -= 1 => @var = @var - 1 @var ||= 1 => @var = @var || 1 @var = @var || 1

That means that, if @var is set, it does not change its value, but if @var is nil, it will set its value with the one you provided. It works like a cache, if there is a value, use it, but if there is not, set the new value and use it.

Try this on the terminal:

[1] pry(main)> var ||= 10
=> 10
[2] pry(main)> var ||= 20
=> 10
[3] pry(main)> var = nil
=> nil
[4] pry(main)> var ||= 20
=> 20