Binary search

gistfile1.js

function split(arr) {
  var l = arr.length,
      m = Math.round(l / 2);
  return {
    left: arr.slice(0, m),
    right: arr.slice(m, l)
  }
}

function find(arr, el) {
  if(arr.length === 1) {
    if(arr[0] === el) return el;
    return -1;
  }
  var s = split(arr);
  if(s.right[0] <= el) return find(s.right, el);
  return find(s.left, el);
}