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var reducePolyfill = function(callback) {  
  var k = [], key, t = this;
  for(key in t) {
    if (hasOwnProperty.call(t, key)) {
      k.push(key);
    }
  }
  var isArray = Object.prototype.toString.call(t) === '[object Array]';
  var len = k.length >>> 0;
  var argLen = arguments.length;
    
  if (t === 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 t.pop();
  }
  
  if (len === 0 && argLen === 2) {
    return arguments[1];
  }
  
  var init, i;
  if (argLen === 1) {
    init = t[k[0]];
    i = 1;
  } else {
    init = arguments[1];
    i = 0;
  }
  
  for(;i<len;i++) {
    init = callback(init, t[k[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));