Using reduce to define map [JS, Python]

MapUsingReduce.js

// In a language with arrays, `reduce` can be used
// to accumulate arrays and thus can be used to
// define array-producing operators like `map`.
Array.prototype.map = function(fn) {
  return this.reduce(function(sum, cur) {
    return sum.concat(fn(cur));
  }, []);
};

MapUsingReduce.py

# In a language with lists, `reduce` can be used
# to accumulate lists and thus can be used to
# define list-producing operators like `map`.
def map(fn, lst):
  return reduce(lambda sum, cur: sum + [fn(cur)], lst, [])