reduceの使い方。汎用的な関数を作る時に超便利。

gistfile1.js

var sum = function(a) {
  return a.reduce(function(x, y) { return x + y; });
};

var max = function(a) {
  return a.reduce(function(x, y) {
    if (x > y) return x;
    return y;
  });
};

var min = function(a) {
  return a.reduce(function(x, y) {
    if (x > y) return y;
    return x;
  });
};

var average = function(a) {
  return a.reduce(function(x, y) {
    if (y === a[a.length-1]) return (x + y) / a.length;
    return x + y;
  });
};

var filter = function(a) {
  a.unshift([]);
  return a.reduce(function(x, y) {
    if (y) {
      x.push(y);
      return x;
    }
    return x;
  });
};

var exist = function(a, val) {
  return a.reduce(function(x, y) {
    return (x === true || x === val || y === val) ? true : false;
  });
};