Array.reduce polyfill created by MDN, made available to use with NPM

array.reduce-polyfill.js

// Production steps of ECMA-262, Edition 5, 15.4.4.21
// Reference: http://es5.github.io/#x15.4.4.21
if (!Array.prototype.reduce) {
  Array.prototype.reduce = function(callback /*, initialValue*/) {
    'use strict';
    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');
    }
    var t = Object(this), len = t.length >>> 0, k = 0, value;
    if (arguments.length == 2) {
      value = arguments[1];
    } else {
      while (k < len && !(k in t)) {
        k++; 
      }
      if (k >= len) {
        throw new TypeError('Reduce of empty array with no initial value');
      }
      value = t[k++];
    }
    for (; k < len; k++) {
      if (k in t) {
        value = callback(value, t[k], k, t);
      }
    }
    return value;
  };
}

array.reduce-polyfill.min.js

Array.prototype.reduce||(Array.prototype.reduce=function(r){"use strict";if(null==this)throw new TypeError("Array.prototype.reduce called on null or undefined");if("function"!=typeof r)throw new TypeError(r+" is not a function");var e,t=Object(this),n=t.length>>>0,o=0;if(2==arguments.length)e=arguments[1];else{for(;n>o&&!(o in t);)o++;if(o>=n)throw new TypeError("Reduce of empty array with no initial value");e=t[o++]}for(;n>o;o++)o in t&&(e=r(e,t[o],o,t));return e});

package.json

{
  "name": "array-reduce-polyfill",
  "version": "1.0.0",
  "main": "array.reduce-polyfill.min.js"
}

why are we checking again (k in t) in last for loop ?