[рони]

Hints:
this[i]
arguments.length

<script src="http://stacksnippets.net/scripts/snippet-javascript-console.min.js?v=1"></script>

  <script>
Array.prototype.reduce2 = function(c, a) {
    var i = 0;
    for (arguments.length == 1 && (a = this[i], i = 1); i < this.length; i++) a = c(a, this[i]);
    return a
};
 function add(a, b) { return a + b }
 function mul(a, b) { return a * b }
 function foo(a, b) { return a.concat(b) }
 var a = [1, 2, 3, 4]
 console.log(a.reduce(add), a.reduce2(add)) // 10 10
 console.log(a.reduce(add, 10), a.reduce2(add, 10)) // 20 20
 console.log(a.reduce(mul), a.reduce2(mul)) // 24 24
 console.log(a.reduce(foo, ''), a.reduce2(foo, '')) // 1234 1234

  </script>