JavaScriptにもflatMapを！ ref: http://qiita.com/alucky0707/items/88bd35208b12243768ac

func_flatMap.js

function flatMap(xs, f) {
  return xs.reduce(function (ys, x) {
    return ys.concat(f(x));
  }, []);
}

//=> [4, 12, 20, 28, 36]
console.log(flatMap([1,2,3,4,5,6,7,8,9,10], function (x) {
  return x % 2 === 1 ? [x << 2] : [];
}));

js;underscore_flatMap.js

var
_ = require('underscore');

var
flatMap = _.flatMap = _.concatMap = function (xs, f, self) {;
  return _.reduce(xs, function (ys, x) {
    return ys.concat(f.call(self, x));
  }, []);
};

_.each(['flatMap', 'concatMap'], function (name) {
  _.prototype[name] = function (f, self) {
    return _(flatMap(this.value(), f, self));
  };
});

//さっきよりシンプルになったように見えるのは_.rangeのおかげ？
console.log(_.flatMap(_.range(1, 11), function (x) {
  return x % 2 === 1 ? [x << 2] : [];
}));

//こういう風にも書ける
console.log(_(_.range(1, 11)).flatMap(function (x) {
  return x % 2 === 1 ? [x << 2] : [];
}).value());

prototype_flatMap.js

Object.defineProperty(Array.prototype, 'flatMap', {
  value: function (f, self) {
    self = self || this;
    return this.reduce(function (ys, x) {
      return ys.concat(f.call(self, x));
    }, []);
  },
  enumerable: false,
});

//=> [4, 12, 20, 28, 36]
console.log([1,2,3,4,5,6,7,8,9,10].flatMap(function (x) {
  return x % 2 === 1 ? [x << 2] : [];
}));