class Opal::Rewriters::ForRewriter

def self.next_tmp 

def self.next_tmp
  @counter ||= 0
  @counter += 1
  :"$for_tmp#{@counter}"
end

def self.reset_tmp_counter! 

def self.reset_tmp_counter!
  @counter = 0
end

def on_for(node)


[[1, 2], [3, 4]].each { |__jstmp| i, j = __jstmp }
j = nil
i = nil
Becomes multiple left-hand assignment:
for i, j in [[1, 2], [3, 4]]; end
Complex stuff with multiple loop variables:

(0..3).each { |__jstmp| i = __jstmp; j = i + 1 }
i = nil
j = nil
This class rewrites it to the following code:

I.e. after this loop variable `i` is 3 and `j` is 4
loop variable + all local variables available outside.
The problem here is that in Ruby for loop makes its

for i in 0..3; j = i + 1; end
Handles 
def on_for(node)
  loop_variable, iterating_value, loop_body = *node
  iterating_lvars        = LocalVariableAssigns.find(loop_variable) # [:i]
  lvars_declared_in_body = LocalVariableAssigns.find(loop_body)     # [:j]
  # i = nil; j = nil
  outer_assigns = (iterating_lvars + lvars_declared_in_body).map do |lvar_name|
    s(:lvdeclare, lvar_name)
  end
  # :__jstmp
  tmp_loop_variable = self.class.next_tmp
  get_tmp_loop_variable = s(:js_tmp, tmp_loop_variable)
  loop_variable_assignment = case loop_variable.type
                             when :mlhs # multiple left-hand statement like in "for i,j in [[1, 2], [3, 4]]"
                               # i, j = __jstmp
                               loop_variable.updated(:masgn, [loop_variable, get_tmp_loop_variable])
                             else # single argument like "for i in (0..3)"
                               # i = __jstmp
                               loop_variable << get_tmp_loop_variable
                             end
  loop_body = prepend_to_body(loop_body, loop_variable_assignment)
  node = node.updated(:send, [iterating_value, :each,                                    # (0..3).each {
                              node.updated(:iter, [s(:args, s(:arg, tmp_loop_variable)), #                |__jstmp|
                                                   process(loop_body)]                   #                          i = __jstmp; j = i + 1 }
                              )]
  )
  node.updated(:begin, [*outer_assigns, node])
end