var reducePolyfill = function(callback) {
  var isArray = Array.isArray(this);
  var t = Object(this);
  var k = Object.keys(this);
  var len = isArray ? t.length >>> 0 : k.length >>> 0;
  var argLen = arguments.length;
  var init;
  var i;

  if (this === null) {
    throw new TypeError('Array.prototype.reduce called on null or undefined');
  }

  if (typeof callback !== 'function') {
    throw new TypeError(callback + ' is not a function');
  }

  if (len === 0 && argLen === 1) {
    throw new TypeError('Reduce of empty array with no initial value');
  }

  if (len === 1 && argLen === 1) {
    return this.pop();
  }

  if (len === 0 && argLen === 2) {
    return arguments[1];
  }

  if (argLen === 1) {
    init = isArray ? t[0] : t[k[0]];
    i = 1;
  } else {
    init = arguments[1];
    i = 0;
  }

  for(;i<len;i++) {
    init = callback(init, t[k[i]], isArray ? i : k[i]);
  }

  return init;
};

Array.prototype.reduce = reducePolyfill;
Object.prototype.reduce = reducePolyfill;

console.log({a: 1, b: 2, c: 3}.reduce(function(res, val) {
  res += val;
  return res;
}, 0));

console.log({a: 1, b: 2, c: 3}.reduce(function(res, val, key) {
  res += `${key}: ${val}\n`;
  return res;
}, ''));

console.log([1, 2, 3].reduce(function(res, val) {
  res += val;
  return res;
}, 0));

console.log([1, 2, 3].reduce(function(res, val, key) {
  res += `${key}: ${val}\n`;
  return res;
}, ''));

console.log([1, 2, 3].reduce(function(res, val, key) {
  res += val;
  return res;
}));

console.log([1].reduce(function(res, val, key) {
  res += val;
  return res;
}));

console.log([].reduce(function(res, val, key) {
  res += val;
  return res;
}, 1));