polyfill of reduce method in javascript

reducePolyfill.js

Array.prototype.myReduce = function() {
    var arr = this;
    var len = arr.length;
    var fn = arguments[0];
    var i = 0;
    var acc = arguments[1];
    if (arguments.length < 2) {
 
        i = 1;
        acc = arr[0];
    }

    for (; i < len; i++) {

        acc = fn(acc, arr[i], i, arr)

    }
    return acc;

}

var arr = [1, 2, 3, 4, 5, 6, 7, 8];

var myReduce = arr.myReduce((prev,item)=>prev + item);
var reduce = arr.reduce((prev,item)=>prev + item);

console.log(myReduce);
// 36
console.log(reduce);
// 36