flatMap.js

flatMap.js

/*
 * flatMap can be implemented in terms of flatten and map
 * which can be implemented using reduce
 */
'use strict';

// > reduce(function(sum, value) { return sum + value; },
// .   0, [1,2,3])
// 6
var reduce = function(iter, initial, arr) {
  if (!arr.length) {
    return initial;
  }

  var newInitial = iter(initial, arr[0]);
  return reduce(iter, newInitial, arr.slice(1));
};

// > map(function(a) { return a + 1; }, [1, 2, 3])
// [2, 3, 4]
// > map(function(a) { return a; }, [])
// []
var map = function(f, arr) {
  return reduce(function(memo, item) {
    return memo.concat([f(item)]);
  }, [], arr);
};

// > concat([1, 2, 3], [4, 5, 6]);
// [1, 2, 3, 4, 5, 6]
var concat = function(arr1, arr2) {
  return reduce(function(memo, item) {
    return memo.concat([item]);
  }, arr1, arr2);
};

// > flatten([[1], [2, 3], [4, 5, 6]])
// [1, 2, 3, 4, 5, 6]
var flatten = function(arrs) {
  return reduce(concat, [], arrs);
};

// > flatMap(function(x) {
// .   return [x, x + 1, x + 2];
// . }, [1, 2, 3]);
// [1, 2, 3, 2, 3, 4, 3, 4, 5]
var flatMap = function(f, arr) {
  return flatten(map(f, arr));
};