Question

I’ve been reading Ruby Refactoring book (Fields, Harvie, Fowler). They mention Extract Surrounding Method operation which can be used to avoid duplication if you have methods which middle part is different from each other.

def number_of_descendants_named(name)
  count_descendants_matchin { |descendant| descendant.name == name }
end

def number_of_living_descendants
  count_descendants_matching { |descendant| descendant.alive? }
end

def count_descendants_mathing(&block)
  children.inject(0) do |count, child|
    count += 1 if yield child
    count + child.count_descendants_matching(&block)
  end
end

I’m sure you get the idea. How would you do similar with Javascript?

Answer 1

Javascript has also closures, so it’s pretty easy, just convert blocks into anonymous functions and the code is practically the same:

var number_of_descendants_named = function(name) {
  return count_descendants_matching(function(descendant) {
    return descendant.name == name;
  });
};

var number_of_living_descendants = function(descendant) {
  return count_descendants_matching(function(descendant) {
    return descendant.alive();
  });
};

var count_descendants_mathing = function(cb) {
  // reduce: use underscore or any other functional library
  return somelib.reduce(children, 0, function(count, child) {
    return count + (cb(child) ? 1 : 0) + child.count_descendants_matching(cb)
  });
};

This functional style where everything is a expression to be returned is very verbose in plain Javascript, but some altJS languages (for example Coffeescript) simplify it a lot.