Search a sorted array in js. O(log n)

search-array.js

#!/usr/bin/env node
var util = require("util");
util.puts("Search Array for value N.");
// This only took me 12 minutes to do on a white board.
// This has a complexity of O(log n).

//        0  1  2  3  4  5  6  7  8   9   10  11
var a = [ 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16 ];

var _indexOf = function(pos1, pos2, n) {
	var middle = pos2 - Math.round((pos2 - pos1) / 2);
	if (n === a[middle]) {
		return middle;
	} else if (n > a[middle]) {
		return _indexOf(middle, pos2, n);
	} else if (n < a[middle]) {
		return _indexOf(pos1, middle, n);
	}
};
var indexOf = function(a, n) {
	return _indexOf(0, a.length, n);
};

util.puts("In array a, indexOf 1 is " + indexOf(a, 1));
util.puts("In array a, indexOf 2 is " + indexOf(a, 2));
util.puts("In array a, indexOf 12 is " + indexOf(a, 12));
util.puts("In array a, indexOf 16 is " + indexOf(a, 16));