ruby_interview https://www.toptal.com/ruby/interview-questions https://www.toptal.com/ruby-on-rails/interview-questions

1

Is the line of code below valid Ruby code? If so, what does it do? Explain your answer.

-> (a) {p a}["Hello world"]

10

In what order will the numbers 1-4 be logged to the console when the code below is executed? Why?

(function() {
    console.log(1); 
    setTimeout(function(){console.log(2)}, 1000); 
    setTimeout(function(){console.log(3)}, 0); 
    console.log(4);
})();

10 answer

The values will be logged in the following order:

1
4
3
2
Let’s first explain the parts of this that are presumably more obvious:

1 and 4 are displayed first since they are logged by simple calls to console.log() without any delay

2 is displayed after 3 because 2 is being logged after a delay of 1000 msecs (i.e., 1 second) whereas 3 is being logged after a delay of 0 msecs.

OK, fine. But if 3 is being logged after a delay of 0 msecs, doesn’t that mean that it is being logged right away? And, if so, shouldn’t it be logged before 4, since 4 is being logged by a later line of code?

The answer has to do with properly understanding JavaScript events and timing.

The browser has an event loop which checks the event queue and processes pending events. For example, if an event happens in the background (e.g., a script onload event) while the browser is busy (e.g., processing an onclick), the event gets appended to the queue. When the onclick handler is complete, the queue is checked and the event is then handled (e.g., the onload script is executed).

Similarly, setTimeout() also puts execution of its referenced function into the event queue if the browser is busy.

When a value of zero is passed as the second argument to setTimeout(), it attempts to execute the specified function “as soon as possible”. Specifically, execution of the function is placed on the event queue to occur on the next timer tick. Note, though, that this is not immediate; the function is not executed until the next tick. That’s why in the above example, the call to console.log(4) occurs before the call to console.log(3) (since the call to console.log(3) is invoked via setTimeout, so it is slightly delayed).

2

Given:

x = "hello"
Explain the difference between:

x += " world"
and

x.concat " world"

3

Consider the following code:

class A
  def self.a(b)
    if b > 0
      b * b
    end
  end
end
What will be the values of:

var1 = A.a(0)
var2 = A.a(2)

4

Can you call a private method outside a Ruby class using its object?

5

What is the difference between Array#map and Array#each?

6

What is the primary difference in these two code snippets?
// Java
public boolean isEmpty(String s) {
  return s.length() == 0;
}
# ruby
def empty?(s)
  return s.size == 0
end

7

What’s the issue with the controller code below? How would you fix it?

class CommentsController < ApplicationController
  def users_comments
    posts = Post.all
    comments = posts.map(&:comments).flatten
    @user_comments = comments.select do |comment|
      comment.author.username == params[:username]
    end
  end
end

8

What paths (HTTP verb and URL) will be defined by the following snippet in config/routes.rb?

resources :posts do
  member do
    get 'comments'
  end
  collection do
    post 'bulk_upload'
  end
end

9

Suppose we have a Student with id=”4”. If we delete the Student with id=”4”, what will be the result of the following queries:

Student.find(4)
Student.find_by_id(4)