Quiz

quiz.md

Questions

ruby

1. inheritance vs nested classes
2. the tap function
3. the reduce function
4. iterator shorthands
5. constant lookup
6. operator precedence
7. requiring files
8. Default hash values
9. re-implement Proc#new
10. index_by vs group_by
11. break vs return
12. safe navigation

javascript/coffeescript

1. NaN
2. more bullshit
3. classes and arrow functions
4. objects and arrow functions
5. the 'return' keyword
6. async loops

---

inheritance vs nested classes

What is the return value or error when calling Foo::Bar.b for each of the following:

1

class Foo
  def self.a; 1; end
  class Bar < Foo
    def self.b; a; end
  end  
end  

2

class Foo
  def self.a; 1; end
  class Bar
    def self.b; a; end
  end  
end  

answer

1. 1
2. NoMethodError

---

the tap function

What are the return values for each of the following:

1

1.tap { |num| num + 1 }

2

1.tap { |num| num += 1 }

3

{}.tap { |hash| hash[:foo] = :bar }

4

{}.tap { |hash| hash.merge foo: :bar }

answer

1. 1
2. 1
3. { foo: :bar }
4. { }

---

the reduce function

What are the return values when calling map_keys({1 => 2}) { |key| key + 1 } with each of the following:

1

  def map_keys(hash, &blk)
    hash.reduce({}) do |memo, (key, val)|
      memo.tap { memo[blk.call key] = val }
    end
  end

2

  def map_keys(hash, &blk)
    hash.reduce({}) do |memo, (key, val)|
      memo[blk.call key] = val
    end
  end

3

  def map_keys(hash, &blk)
    hash.each_with_object({}) do |(key, val), memo|
      memo[blk.call key] = val
    end
  end

answer

1. { 2 => 2 }
2. 2
3. { 2 => 2 }

---

iterator shorthands

What will be printed by each of the following (ignore the return vals), or will there be an error

1

[[]].map(&:class).each &method(:print)

2

[[]].map(&:class).each &:print

3

 [[]].map { |elem| elem.class }.each { |elem| print elem }

answer

1. Array
2. NoMethodError
3. Array

--

Constant lookup

Given class Foo; class Bar; end; end, what is the result or error of each of the following:

1

Foo.class_exec { Bar }

2

Foo.class_exec { ::Bar }

3

Foo.class_exec { self::Bar }

answer

1. NameError
2. NameErrr
3. Bar

---

operator precedence

What will the following return?

1

false if true && true

2

nil if true && true

answer

1. false
2. nil

---

requiring files

Say there are three files, each containing one of the following code snippets. For each of the files, what is the result or error of requiring it from pry and entering foo?

1

foo = :bar

2

def foo; :bar; end

3

foo = ->{ :bar }

answer

1. NoMethodError
2. :bar
3. NoMethodError

---

Default hash values

What is the result of each of the following?

1

Hash.new(0)[:a]

2

Hash.new(0).has_key? :a

3

Hash.new(:a).has_key? :a

4

Hash.new { |h,k| h[k] = :a }[:a]

answer

1. 0
2. false
3. false
4. :a

---

re-implement Proc#new

Proc#new is used like this:

fn = Proc.new { |arg| arg + 1 }
fn.call(1) # => 2

How would you implement the Proc class to have this behaviour?

answer

    class Proc
      def initiailize(&blk)
        @blk = blk
      end
      def call
        @blk.call
      end
    end
  

---

index_by vs group_by

What is the output of each of the following?

1

[1,2,3,4].index_by { |x| x % 2 == 0 }

2

[1,2,3,4].group_by { |x| x % 2 == 0 }

answer

1. { false => 3, true => 4 }
2. { false => [1,3], true => [2,4] }

---

break vs returnn

What will be printed by the following?

1

def foo
  loop { puts "1"; loop { puts "2"; break }; puts "3"; break }
end

2

def foo
  loop { print "1"; loop { print "2"; return }; print "3"; break }
end

answer

1. 123
2. 12

---

safe navigation

What will be printed by the following?

1

nil&.puts(puts(1).class)

answer

Nothing

---

---

NaN

What is the result of each of the following (coffeescript):

1

NaN == NaN

2

typeof NaN

3

isNaN NaN

answer

1. false
2. 'number'
3. true

---

more bullshit

What is the result (or error) of each of the following (coffeescript):

1

typeof null

2

typeof undefined

3

{}.constructor()

4

x = {}
x.constructor()

5

!0

answer

1. 'object'
2. 'object'
3. error (constructor is not a function)
4. Object
5. true

---

classes and arrow functions

Given the following coffeescript:

class A
  b: -> @d
  c: => @d
  d: "ok"

a = new A()

What will each of the following return:

1

a.b() == "ok"

2

a.c() == "ok"

3

a.b.apply({}) == "ok"

4

a.c.apply({}) == "ok"

answer

1. true
2. true
3. false
4. true

---

objects and arrow functions

Given the following coffeescript:

a = 
  b: -> @d
  c: => @d
  d: "ok"

What will each of the following return:

1

a.b() == "ok"

2

a.c() == "ok"

3

a.b.apply({}) == "ok"

4

a.c.apply({}) == "ok"

answer

1. true
2. false
3. false
4. false

---

the 'return' keyword

What is the return value of x() for the following function?

x = ->
  [1,2,3].map (i) ->
    return i if i % 2 == 0
    null

answer

[null, 2, null]

---

async loops

Give the following sleep function:

sleep_1_sec = ->
  new Promise (resolve) ->
    setTimeout resolve, 1000

Which of the following countdown functions will work correctly?

1

countdown = ->
  for i in [1,2,3]
    await sleep_1_sec()
    console.log i
countdown()

2

countdown = ->
  [1,2,3].forEach (i) ->
    await sleep_1_sec()
    console.log(i)
countdown()

answer

1. correct (prints numbers with 1-second sleep in between)
2. incorrect (prints all numbers at once after 1 second)